ตัวอย่างข้อสอบ gmat

ตัวอย่างข้อสอบ gmat

INEQUALITIES

Inequalities are manipulated algebraically the same way as equations with one exception:

Multiplying or dividing both sides of an inequality by a negative number reverses the inequality. That is, if x > y and c <>

Example: For which values of x is 4x + 3 > 6x - 8?

As with equations, our goal is to isolate x on one side:

Subtracting 6x from both sides yields -2x + 3 > -8

Subtracting 3 from both sides yields -2x > -11

Dividing both sides by -2 and reversing the inequality yields x < 11/2

Positive & Negative Numbers

A number greater than 0 is positive. On the number line, positive numbers are to the right of 0. A number less than 0 is negative. On the number line, negative numbers are to the left of 0. Zero is the only number that is neither positive nor negative; it divides the two sets of numbers. On the number line, numbers increase to the right and decrease to the left.

The expression x > y means that x is greater than y. In other words, x is to the right of y on the number line.

We usually have no trouble determining which of two numbers is larger when both are positive or one is positive and the other negative (e.g., 5 > 2 and 3.1 > -2). However, we sometimes hesitate when both numbers are negative (e.g., -2 > -4.5). When in doubt, think of the number line: if one number is to the right of the number, then it is larger.

Miscellaneous Properties of Positive and Negative Numbers

1. The product (quotient) of positive numbers is positive.
2. The product (quotient) of a positive number and a negative number is negative.
3. The product (quotient) of an even number of negative numbers is positive.
4. The product (quotient) of an odd number of negative numbers is negative.
5. The sum of negative numbers is negative.
6. A number raised to an even exponent is greater than or equal to zero.

Absolute Value

The absolute value of a number is its distance on the number line from 0. Since distance is a positive number, absolute value of a number is positive. Two vertical bars denote the absolute value of a number: | x |. For example, | 3 | = 3 and | -3 | = 3.

Students rarely struggle with the absolute value of numbers: if the number is negative, simply make it positive; and if it is already positive, leave it as is. For example, since -2.4 is negative, | -2.4 | = 2.4 and since 5.01 is positive | 5.01 | = 5.01.

Further, students rarely struggle with the absolute value of positive variables: if the variable is positive, simply drop the absolute value symbol. For example, if x > 0, then | x | = x.

However, negative variables can cause students much consternation. If x is negative, then | x | = -x. This often confuses students because the absolute value is positive but the -x appears to be negative. It is actually positive--it is the negative of a negative number, which is positive. To see this more clearly let x = -k, where k is a positive number. Then x is a negative number. So | x | = -x = -(-k) = k. Since k is positive so is -x. Another way to view this is | x | = -x = (-1)x = (-1)(a negative number) = a positive number.

Transitive Property

If x <>

Example: If 1/Q > 1, is 1 > QQ ?

Since 1/Q > 1 and 1 > 0, we know from the transitive property that 1/Q is positive. Hence, Q is positive. Therefore, we can multiply both sides of 1/Q > 1 by Q without reversing the inequality:

Q(1/Q) > 1(Q)
Reducing yields 1 > Q
Multiplying both sides again by Q yields Q > QQ
Using the transitive property to combine the last two inequalities yields 1 > QQ

FRACTIONS

I. To compare two fractions, cross-multiply. The larger number will be on the same side as the larger fraction.

Example: Example: Which fraction is greater 9/10 or 10/11 ?

Cross-multiplying gives (9)(11) versus (10)(10), which reduces to 99 versus 100. Now, 100 is greater than 99. Hence, 10/11 is greater than 9/10.

III. To solve a fractional equation, multiply both sides by the LCD (lowest common denominator) to clear fractions.

Example: If (x + 3)/(x - 3) = y, what is the value of x in terms of y?

(A) 3 - y (B) 3/y (C) (2 + y)/(y - 2) (D) (-3y -3)/(1 - y) (E) 3y/2

First, multiply both sides of the equation by x - 3: (x - 3)(x + 3)/(x - 3) = (x - 3)y

Cancel the (x - 3's) on the left side of the equation: x + 3 = (x - 3)y

Distribute the y: x + 3 = xy - 3y

Subtract xy and 3 from both sides: x - xy = -3y - 3

Factor out the x on the left side of the equation: x(1 - y) = -3y - 3

Finally, divide both sides of the equation by 1 - y: x = (-3y -3)/(1 - y)

Hence, the answer is (D).

IV. When dividing a fraction by a whole number (or vice versa), you must keep track of the main division bar.

Example: a/(b/c) = a(c/b) = ac/b. But (a/b)/c = (a/b)(1/c) = a/(bc).

V. Two fractions can be added quickly by cross-multiplying: a/b + c/d = (ad + bc)/bd

Example: 1/2 - 3/4 =

(A) -5/4 (B) -2/3 (C) -1/4 (D) 1/2 (E) 2/3

Cross multiplying the expression 1/2 - 3/4 yields [1(4) - 2(3)]/2(4) = (4 - 6)/8 = -2/8 = -1/4. Hence, the answer is (C).

VI. To find a common denominator of a set of fractions, simply double the largest denominator until all the other denominators divide into it evenly.

VII. Fractions often behave in unusual ways: Squaring a fraction makes it smaller, and taking the square root of a fraction makes it larger. (Caution: This is true only for proper fractions, that is, fractions between 0 and 1.)

Example: 1/3 squared equals 1/9 and 1/9 is less than 1/3. Also the square root of 1/4 is 1/2 and 1/2 is greater than 1/4.

AVERAGES

Problems involving averages are very common on the GMAT. They can be classified into four major categories as follows.

I. The average of N numbers is their sum divided by N, that is, average = sum/N.

Example: The average of x, 2x, and 6 is (x + 2x + 6)/3 = (3x + 6)/3 = 3(x + 2)/3 = x + 2.

II. Weighted average: The average between two sets of numbers is closer to the set with more numbers.

Example: If on a test three people answered 90% of the questions correctly and two people answered 80% correctly, then the average for the group is not 85% but rather [3(90) + 2(80)]/5 = 430/5 = 86. Here, 90 has a weight of 3--it occurs 3 times. Whereas 80 has a weight of 2--it occurs 2 times. So the average is closer to 90 than to 80 as we have just calculated.

III. Using an average to find a number.

Sometimes you will be asked to find a number by using a given average. An example will illustrate.

Example: If the average of five numbers is -10, and the sum of three of the numbers is 16, then what is the average of the other two numbers?

(A) -33 (B) -1 (C) 5 (D) 20 (E) 25

Let the five numbers be a, b, c, d, e. Then their average is (a + b + c + d + e)/5 = -10. Now three of the numbers have a sum of 16, say, a + b + c = 16. So substitute 16 for a + b + c in the average above: (16 + d + e)/5 = -10. Solving this equation for d + e gives d + e = -66. Finally, dividing by 2 (to form the average) gives (d + e)/2 = -33. Hence, the answer is (A).

IV. Average Speed = Total Distance/Total Time

Although the formula for average speed is simple, few people solve these problems correctly because most fail to find both the total distance and the total time.

Example: In traveling from city A to city B, John drove for 1 hour at 50 mph and for 3 hours at 60 mph. What was his average speed for the whole trip?

(A) 50 (B) 53 1/2 (C) 55 (D) 56 (E) 57 1/2

The total distance is 1(50) + 3(60) = 230. And the total time is 4 hours. Hence, Average Speed = Total Distance/Total Time = 230/4 = 57 1/2. The answer is (E). Note, the answer is not the mere average of 50 and 60. Rather the average is closer to 60 because he traveled longer at 60 mph (3 hrs) than at 50 mph (1 hr).

RATIO & PROPORTION

Ratio

A ratio is simply a fraction. Both of the following notations express the ratio of x to y: x:y, x/y. A ratio compares two numbers. Just as you cannot compare apples and oranges, so too must the numbers you are comparing have the same units. For example, you cannot form the ratio of 2 feet to 4 yards because the two numbers are expressed in different units--feet vs. yards. It is quite common for the GMAT to ask for the ratio of two numbers with different units. Before you form any ratio, make sure the two numbers are expressed in the same units.

Proportion

A proportion is simply an equality between two ratios (fractions). For example, the ratio of x to y is equal to the ratio of 3 to 2 is translated as x/y = 3/2. Two variables are directly proportional if one is a constant multiple of the other:

y = kx, where k is a constant.

The above equation shows that as x increases (or decreases) so does y. This simple concept has numerous applications in mathematics. For example, in constant velocity problems, distance is directly proportional to time: d = vt, where v is a constant. Note, sometimes the word directly is suppressed.

Example: If the ratio of y to x is equal to 3 and the sum of y and x is 80, what is the value of y?

(A) -10 (B) -2 (C) 5 (D) 20 (E) 60

Translating "the ratio of y to x is equal to 3" into an equation yields: y/x = 3

Translating "the sum of y and x is 80" into an equation yields: y + x = 80

Solving the first equation for y gives: y = 3x.

Substituting this into the second equation yields

3x + x = 80
4x = 80
x = 20

Hence, y = 3x = 3(20) = 60. The answer is (E).

In many word problems, as one quantity increases (decreases), another quantity also increases (decreases). This type of problem can be solved by setting up a direct proportion.

Example: If Biff can shape 3 surfboards in 50 minutes, how many surfboards can he shape in 5 hours?

(A) 16 (B) 17 (C) 18 (D) 19 (E) 20

As time increases so does the number of shaped surfboards. Hence, we set up a direct proportion. First, convert 5 hours into minutes: 5 hours = 5 x 60 minutes = 300 minutes. Next, let x be the number of surfboards shaped in 5 hours. Finally, forming the proportion yields

3/50 = x/300
3(300)/50 = x
18 =x

The answer is (C).

If one quantity increases (or decreases) while another quantity decreases (or increases), the quantities are said to be inversely proportional. The statement "y is inversely proportional to x" is written as

y = k/x, where k is a constant.

Multiplying both sides of y = k/x by x yields

yx = k

Hence, in an inverse proportion, the product of the two quantities is constant. Therefore, instead of setting ratios equal, we set products equal.

In many word problems, as one quantity increases (decreases), another quantity decreases (increases). This type of problem can be solved by setting up a product of terms.

Example: If 7 workers can assemble a car in 8 hours, how long would it take 12 workers to assemble the same car?

(A) 3hrs (B) 3 1/2hrs (C) 4 2/3hrs (D) 5hrs (E) 6 1/3hrs

As the number of workers increases, the amount time required to assemble the car decreases. Hence, we set the products of the terms equal. Let x be the time it takes the 12 workers to assemble the car. Forming the equation yields

7(8) = 12x
56/12 = x
4 2/3 = x

The answer is (C).

To summarize: if one quantity increases (decreases) as another quantity also increases (decreases), set ratios equal. If one quantity increases (decreases) as another quantity decreases (increases), set products equal.

EXPONENTS & ROOTS

Exponents

There are five rules that govern the behavior of exponents:

Problems involving these five rules are common on the GMAT, and they are often listed as hard problems. However, the process of solving these problems is quite mechanical: simply apply the five rules until they can no longer be applied.

Roots

There are only two rules for roots that you need to know for the GMAT:

FACTORING

To factor an algebraic expression is to rewrite it as a product of two or more expressions, called factors. In general, any expression on the GMAT that can be factored should be factored, and any expression that can be unfactored (multiplied out) should be unfactored.

Distributive Rule

The most basic type of factoring involves the distributive rule:
ax + ay = a(x + y)

For example, 3h + 3k = 3(h + k), and 5xy + 45x = 5xy + 9(5x) = 5x(y + 9). The distributive rule can be generalized to any number of terms. For three terms, it looks like ax + ay + az = a(x + y + z). For example, 2x + 4y + 8 = 2x + 2(2y) + 2(4) = 2(x + 2y + 4).

Example: If x - y = 9, then (x - y/3) - (y - x/3) =

(A) -4 (B) -3 (C) 0 (D) 12 (E) 27

(x - y/3) - (y - x/3) =
x - y/3 - y + x/3 =
4x/3 - 4y/3 =
4(x - y)/3 =
4(9)/3 =
12

The answer is (D).

Difference of Squares

One of the most important formulas on the GMAT is the difference of squares:

Example: If x does not equal -2, then

(A) 2(x - 2) (B) 2(x - 4) (C) 8(x + 2) (D) x - 2 (E) x + 4

In most algebraic expressions involving multiplication or division, you won't actually multiply or divide, rather you will factor and cancel, as in this problem.

2(x - 2)

The answer is (A).

Perfect Square Trinomials

Like the difference of squares formula, perfect square trinomial formulas are very common on the GMAT.

For example,.

ALGEBRAIC EXPRESSIONS

A mathematical expression that contains a variable is called an algebraic expression. Some examples of algebraic expressions are 3x - 2y, 2z/y. Two algebraic expressions are called like terms if both the variable parts and the exponents are identical. That is, the only parts of the expressions that can differ are the coefficients. For example, x + y and -7(x + y) are like terms. However, x - y and 2 - y are not like terms.

Adding & Subtracting Algebraic Expressions

Only like terms may be added or subtracted. To add or subtract like terms, merely add or subtract their coefficients:

You may add or multiply algebraic expressions in any order. This is called the commutative property:
x + y = y + x xy = yx

For example, -2x + 5x = 5x + (-2x) = (5 - 2)x = -3x and (x - y)(-3) = (-3)(x - y) = (-3)x - (-3)y = -3x + 3y.

Caution: the commutative property does not apply to division or subtraction.

When adding or multiplying algebraic expressions, you may regroup the terms. This is called the associative property:
x + (y + z) = (x + y) + z x(yz) = (xy)z

Notice in these formulas that the variables have not been moved, only the way they are grouped has changed: on the left side of the formulas the last two variables are grouped together, and on the right side of the formulas the first two variables are grouped together.

For example, (x -2x) + 5x = (x + [-2x]) + 5x = x + (-2x + 5x) = x + 3x = 4x and 24x = 2x(12x) = 2x(3x4x) = (2x3x)4x = 6x4x = 24x

Caution: the associative property doesn't apply to division or subtraction.

Notice in the first example that we changed the subtraction into negative addition: (x - 2x) = (x + [- 2x]). This allowed us to apply the associative property over addition.

Parentheses

When simplifying expressions with nested parentheses, work from the inner most parentheses out:

5x + (y - (2x - 3x)) = 5x + (y - (-x)) = 5x + (y + x) = 6x + y

Sometimes when an expression involves several pairs of parentheses, one or more pairs are written as brackets. This makes the expression easier to read:

2x(1 -[y + 2(3 - y)]) =
2x(1 -[y + 6 - 2y]) =
2x(1 -[-y + 6]) =
2x(1 + y - 6) =
2x(y - 5) =
2xy - 10x

Order of Operations: (PEMDAS)

When simplifying algebraic expressions, perform operations within parentheses first and then exponents and then multiplication and then division and then addition and then subtraction. This can be remembered by the mnemonic:
PEMDAS Please Excuse My Dear Aunt Sally

GRAPHS

Questions involving graphs rarely involve any significant calculating. Usually, the solution is merely a matter of interpreting the graph.

1. During which year was the company's earnings 10 percent of its sales?

(A) 85 (B) 86 (C) 87 (D) 88 (E) 90

Reading from the graph, we see that in 1985 the company's earnings were $8 million and its sales were $80 million. This gives 8/80 = 1/10 = 10/100 = 10%. The answer is (A).

2. During what two-year period did the company's earnings increase the greatest?

(A) 85-87 (B) 86-87 (C) 86-88 (D) 87-89 (E) 88-90

Reading from the graph, we see that the company's earnings increased from $5 million in 1986 to $10 million in 1987, and then to $12 million in 1988. The two-year increase from '86 to '88 was $7 million--clearly the largest on the graph. The answer is (C).

3. During the years 1986 through 1988, what were the average earnings per year?

(A) 6 million (B) 7.5 million (C) 9 million (D) 10 million (E) 27 million

The graph yields the following information:

Year	Earnings
1986	$5 million
1987	$10 million
1988	$12 million

Forming the average yields (5 + 10 + 12)/3 = 27/3 = 9. The answer is (C).

4. If Consolidated Conglomerate's earnings are less than or equal to 10 percent of sales during a year, then the stockholders must take a dividend cut at the end of the year. In how many years did the stockholders of Consolidated Conglomerate suffer a dividend cut?

(A) None (B) One (C) Two (D) Three (E) Four

Calculating 10 percent of the sales for each year yields

Year	10% of Sales (millions)	Earnings (millions)
85	.10 x 80 = 8	8
86	.10 x 70 = 7	5
87	.10 x 50 = 5	10
88	10 x 80 = 8	12
89	.10 x 90 = 9	11
90	.10 x 100 = 10	8

Comparing the right columns shows that earnings were 10 percent or less of sales in 1985, 1986, and 1990. The answer is (D).

WORD PROBLEMS

Although exact steps for solving word problems cannot be given, the following guidelines will help:

(1) First, choose a variable to stand for the least unknown quantity, and then try to write the other unknown quantities in terms of that variable.

For example, suppose we are given that Sue's age is 5 years less than twice Jane's and the sum of their ages is 16. Then Jane's age would be the least unknown, and we let x = Jane's age. Expressing Sue's age in terms of x gives Sue's age = 2x - 5.

(2) Second, write an equation that involves the expressions in Step 1. Most (though not all) word problems pivot on the fact that two quantities in the problem are equal. Deciding which two quantities should be set equal is usually the hardest part in solving a word problem since it can require considerable ingenuity to discover which expressions are equal.

For the example above, we would get (2x - 5) + x = 16.

(3) Third, solve the equation in Step 2 and interpret the result.

For the example above, we would get by adding the x's: 3x - 5 = 16. Then adding 5 to both sides gives 3x = 21. Finally, dividing by 3 gives x = 7. Hence, Jane is 7 years old and Sue is 2x - 5 = 2(7) - 5 = 9 years old.

Motion Problems

Virtually, all motion problems involve the formula Distance = Rate x Time, or

D = R x T

Example: Scott starts jogging from point X to point Y. A half-hour later his friend Garrett who jogs 1 mile per hour slower than twice Scott's rate starts from the same point and follows the same path. If Garrett overtakes Scott in 2 hours, how many miles will Garrett have covered?

(A) 2 1/5 (B) 3 1/3 (C) 4 (D) 6 (E) 6 2/3

Following Guideline 1, we let r = Scott's rate. Then 2r - 1 = Garrett's rate. Turning to Guideline 2, we look for two quantities that are equal to each other. When Garrett overtakes Scott, they will have traveled the same distance. Now, from the formula D = R x T, Scott's distance is D = r x 2 1/2 and Garrett's distance is D = (2r - 1)2 = 4r - 2. Setting these expressions equal to each other gives 4r - 2 = r x 2 1/2. Solving this equation for r gives r = 4/3. Hence, Garrett will have traveled D = 4r - 2 = 4(4/3) - 2 = 3 1/3 miles. The answer is (B).

Work Problems

The formula for work problems is Work = Rate x Time, or W = R x T. The amount of work done is usually 1 unit. Hence, the formula becomes 1 = R x T. Solving this for R gives R = 1/T.

Example : If Johnny can mow the lawn in 30 minutes and with the help of his brother, Bobby, they can mow the lawn 20 minutes, how long would take Bobby working alone to mow the lawn?

(A) 1/2 hour (B) 3/4 hour (C) 1 hour (D) 3/2 hours (E) 2 hours

Let r = 1/t be Bobby's rate. Now, the rate at which they work together is merely the sum of their rates:

Total Rate = Johnny's Rate + Bobby's Rate
1/20 = 1/30 + 1/t
1/20 - 1/30 = 1/t
(30 - 20)/(30)(20) = 1/t
1/60 = 1/t
t = 60

Hence, working alone, Bobby can do the job in 1 hour. The answer is (C).

Mixture Problems

The key to these problems is that the combined total of the concentrations in the two parts must be the same as the whole mixture.

Example : How many ounces of a solution that is 30 percent salt must be added to a 50-ounce solution that is 10 percent salt so that the resulting solution is 20 percent salt?

(A) 20 (B) 30 (C) 40 (D) 50 (E) 60

Let x be the ounces of the 30 percent solution. Then 30%x is the amount of salt in that solution. The final solution will be 50 + x ounces, and its concentration of salt will be 20%(50 + x). The original amount of salt in the solution is 10%(50). Now, the concentration of salt in the original solution plus the concentration of salt in the added solution must equal the concentration of salt in the resulting solution: 10%(50) + 30%x = 20%(50 + x). Multiply this equation by 100 to clear the percent symbol and then solving for x yields x = 50. The answer is (D).

Coin Problems

The key to these problems is to keep the quantity of coins distinct from the value of the coins. An example will illustrate.

Example : Laura has 20 coins consisting of quarters and dimes. If she has a total of $3.05, how many dimes does she have?

(A) 3 (B) 7 (C) 10 (D) 13 (E) 16

Let D stand for the number of dimes, and let Q stand for the number of quarters. Since the total number of coins in 20, we get D + Q = 20, or Q = 20 - D. Now, each dime is worth 10 cents, so the value of the dimes is 10D. Similarly, the value of the quarters is 25Q = 25(20 - D). Summarizing this information in a table yields

	Dimes	Quarters	Total
Number	D	20 - D	20
Value	10D	25(20 - D)	305

Notice that the total value entry in the table was converted from $3.05 to 305 cents. Adding up the value of the dimes and the quarters yields the following equation:

10D + 25(20 - D) = 305
10D + 500 - 25D = 305
-15D = -195
D = 13

Hence, there are 13 dimes, and the answer is (D).

Age Problems

Typically, in these problems, we start by letting x be a person's current age and then the person's age a years ago will be x - a and the person's age a years in future will be x + a. An example will illustrate.

Example : John is 20 years older than Steve. In 10 years, Steve's age will be half that of John's. What is Steve's age?

(A) 2 (B) 8 (C) 10 (D) 20 (E) 25

Steve's age is the most unknown quantity. So we let x = Steve's age and then x + 20 is John's age. Ten years from now, Steve and John's ages will be x + 10 and x + 30, respectively. Summarizing this information in a table yields

	Age now	Age in 10 years
Steve	x	x + 10
John	x + 20	x + 30

Since "in 10 years, Steve's age will be half that of John's," we get

(x + 30)/2 = x + 10
x + 30 = 2(x + 10)
x + 30 = 2x + 20
x = 10

Hence, Steve is 10 years old, and the answer is (C).

ตัวอย่างข้อสอบ gmat