GRE_Math_Bible_eBook

Averages 281

Hard

11.Let the three numbers be x, y, and z. We are given that

x+ y = 2 2

y+ z = 3 2

x + z = 4 2

Summing the three equations yields

x + y

+

y + z

+

x + z

= 2 + 3+ 4

2

2

2

x

+

y

+

y

+

z

+

x

+

z

= 9

2

2

2

2

2

2

x + y + z = 9

The average of the three numbers is (x + y + z)/3 = 9/3 = 3. The answer is (C).

12. Let the number of players on team A be a and the number of players on team B be b. Since the average age of the players on team A is 20, the sum of the ages of the players on the team is 20a. Similarly, since the average age of the players on team B is b, the sum of the ages of the players on the team is 30b.

Now, the average age of the players of the two teams together is

Sum of the ages of the players on each team = Total number of players on the teams

20a + 30b a + b

We are given that this average is 26. Hence, we have

20a + 30b = 26

a + b
20a + 30b = 26a + 26b	by multiplying both sides by a + b
4b = 6a	by subtracting 20a + 26b from both sides
b/a = 6/4 = 3/2

Since a and b are both positive (being the team strengths) and since b/a equals 3/2, which is greater than 1, b must be greater than a. Hence, Column B is greater than Column A, and the answer is (B).

13. Let a, b, and c be the annual incomes of Jack, Jill, and Jess, respectively.

Now, we are given that

The arithmetic mean of the annual incomes of Jack and Jill was $3800. Hence, (a + b)/2 = 3800. Multiplying by 2 yields a + b = 2 3800 = 7600.

The arithmetic mean of the annual incomes of Jill and Jess was $4800. Hence, (b + c)/2 = 4800. Multiplying by 2 yields b + c = 2 4800 = 9600.

The arithmetic mean of the annual incomes of Jess and Jack was $5800. Hence, (c + a)/2 = 5800. Multiplying by 2 yields c + a = 2 5800 = 11,600.

Summing these three equations yields

282 GRE Math Bible

(a + b) + (b + c) + (c + a) = 7600 + 9600 + 11,600 2a + 2b + 2c = 28,800

a + b + c = 14,400

The average of the incomes of the three equals the sum of the incomes divided by 3: (a + b + c)/3 =

14,400/3 = 4800

The answer is (D).

14. Mike ran 10 miles at 10 mph (for the Time = Distance/Rate = 10 miles/10 mph = 1 hour). He ran the remaining 20 miles at 5 mph (for the Time = Distance/Rate = 20 miles/5 mph = 4 hrs). The length of the Marathon track is 30 miles, and the total time taken to cover the track is 5 hours.

Now, let the time taken by Fritz to travel the 30-mile Marathon track be t hours. Then as given, Fritz ran at 10 mph for t/3 hours and at 5 mph for the rest of 2t/3 hours. Hence, by formula, Distance = Rate time, the

	distance covered is (10 mph)	t	+ (5 mph)	2t	10			10		20	t = 30 miles. Solving the equation for t
					=		+		t =	3
		3		3		3		3

yields t = 90/20 = 4.5 hours.

Since Fritz took less time to cover the Marathon than Mike, the average speed of Fritz is greater. Hence, Column B is greater than Column A, and the answer is (B).

15. Column A: The five consecutive integers starting from m are m, m + 1, m + 2, m + 3, and m + 4. The average of the five numbers equals

The sum of the five numbers = 5

m + (m +1) + (m + 2) + (m + 3) + (m + 4) = 5

5m +10 =

5 m + 2

Column B: The six consecutive integers starting from m are m, m + 1, m + 2, m + 3, m + 4, and m + 5. The average of the six numbers equals

The sum of the six numbers = 6

m + (m +1) + (m + 2) + (m + 3) + (m + 4) + (m + 5) = 6

6m +15 =

6

5 m + 2 =

1 m + 2 + 2 =

(m + 2) + 12 = Column A + 12

Since Column B is 1/2 units greater than Column A, the answer is (B).

Averages 283

Method II

Choose any five consecutive integers, say, –2, –1, 0, 1 and 2. (We chose these particular numbers to make the calculation as easy as possible. But any five consecutive integers will do. For example, 1, 2, 3, 4, and

5.) Forming the average yields			1+ ( 2) + 0 + 1+ 2				=	0	= 0. Now, add 3 to the set to form 6 consecutive
					5		5
integers: –2, –1, 0, 1, 2, and 3. Forming the average yields
	1+ ( 2) + 0 + 1+ 2 + 3			=
		6
[ 1+ ( 2) + 0 + 1+ 2] + 3					=
		6
[0] + 3 =		since the average of – 1					+ (–2) + 0 + 1 + 2 is zero, their sum must be zero
6

	3	=
	6
	1
	2

Since 1/2 > 0, Column B is greater than Column A and the answer is (B).

	16. The arithmetic mean of the numbers a and b is 17. Hence,	a + b	= 17.
		2

The geometric mean of the numbers a and b is 8. Hence, ab = 8.

		a + b		a + a
	Now, assume a = b. Then		=		= a = 17. Hence, ab = aa = a = 17 . But, we are given that
		2
			2

ab = 8. Hence, our assumption is wrong. So a b.

Now, suppose the solution for the question is a = s and b = t, and say s is greater than t. Then (s + t)/2 must equal 17 and st must equal 8. Here, Column A = a > b = Column B.

Even a = t and b = s is possible (by substitution we get, (t + s)/2 = 17 and st = 8 ). Here, a < b. Hence, we have a double case, and the answer is (D).

Very Hard

17. Let e be the number of employees.

We are given that 40% of the employees are workers. Now, 40% of e is 40/100 e = 0.4e. Hence, the number of workers is 2e/5.

All the remaining employees are executives, so the number of executives equals (The number of Employees) – (The number of Workers) =

e – 2e/5 = 3e/5

The annual income of each worker is $390. Hence, the total annual income of all the workers together is 2e/5 390 = 156e.

Also, the annual income of each executive is $420. Hence, the total income of all the executives together is 3e/5 420 = 252e.

Hence, the total income of the employees is 156e + 252e = 408e.

The average income of all the employees together equals

284 GRE Math Bible

(The total income of all the employees) ÷ (The number of employees) = 408e/e =

408

The answer is (C).

18. Let a, b, c, d, and e be the five positive numbers in the decreasing order of size such that e is the smallest number. We are given that the average of the five numbers is 25. Hence, we have the equation

a + b + c + d + e = 25 5

a + b + c + d + e = 125 … (1) by multiplying by 5

The smallest number in a set is at least less than the average of the numbers in the set if at least one number is different. For example, the average of 1, 2, and 3 is 2, and the smallest number in the set 1 is less than the average 2. Hence, we have the inequality

0 < e < 25

0 > –e > –25 by multiplying both sides of the inequality by –1 and flipping the directions of the inequalities.

Adding this inequality to equation (1) yields

0 + 125 > (a + b + c + d + e) + (–e) > 125 – 25

125		> a + b + c + d > 100
125		> a + b + c + d + 0 > 100			by adding by 0
25	>		a + b + c + d + 0	= > 20	by dividing the inequality by 5
		5
25	> The average of numbers a, b, c, d and 0 > 20

Hence, Column A equals =

(Average of the numbers a, b, c, d, and e) – (Average of the numbers a, b, c, and d) =

25 – (A number between 20 and 25) =

A number less than 5

Hence, Column A is less than Column B, and the answer is (B).

< 1, we know that 3 is less than . A ratio compares two numbers. Just as you cannot compare apples

Ratio & Proportion

RATIO

A ratio is simply a fraction. The following notations all express the ratio ofx to y: x: y , x y , or x . y

Writing two numbers as a ratio provides a convenient way to compare their sizes. For example, since 3

and oranges, so to 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 GRE 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.

Example 1: What is the ratio of 2 feet to 4 yards?

(A) 1 : 9

(B) 1 : 8

(C) 1 : 7

(D) 1 : 6

(E) 1 : 5

The ratio cannot be formed until the numbers are expressed in the same units. Let’s turn the yards into feet. Since there are 3 feet in a yard, 4 yards = 4 3 feet = 12 feet . Forming the ratio yields

2 feet = 1 or 1 : 6

12 feet 6

The answer is (D).

Note, taking the reciprocal of a fraction usually changes its size. For example, 43 43 . So order is important in a ratio: 3:4 4:3.

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

x3

y= 2

or in ratio notation,

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.

285

286GRE Math Bible

Example 2: 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 yx = 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 3: 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 60 minutes = 300 minutes . Next, let x be the number of

surfboards shaped in 5 hours. Finally, forming the proportion yields

3	=	x
50		300

3 300 = x

50

18 =x

The answer is (C).

Example 4: On a map, 1 inch represents 150 miles. What is the actual distance between two cities if they are 3 12 inches apart on the map?

(A) 225

(B) 300

(C) 450

(D) 525

(E) 600

As the distance on the map increases so does the actual distance. Hence, we set up a direct proportion. Let x be the actual distance between the cities. Forming the proportion yields

1in	3		1	in
	=		2
150 mi			x mi

x= 3 12 150

x= 525

The answer is (D).

Note, you need not worry about how you form the direct proportion so long as the order is the same on both

sides of the equal sign. The proportion in Example 4 could have been written as	1in	=	150 mi	. In this
	3 12 in		x mi

case, the order is inches to inches and miles to miles. However, the following is not a direct proportion

because the order is not the same on both sides of the equal sign:	1in	=		x mi	. In this case, the order
				1
	150 mi
			3	2 in

is inches to miles on the left side of the equal sign but miles to inches on the right side.

Ratio & Proportion 287

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 = kx

where k is a constant.

Multiplying both sides of y = kx 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 5: 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	hrs	(C) 4	2	hrs	(D) 5hrs	(E) 6	1	hrs
		2			3				3

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

7 8 = 12 x

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.

The concept of proportion can be generalized to three or more ratios. A, B, and C are in the ratio 3:4:5 means AB = 34 , CA = 35 , and CB = 45 .

Example 6: In the figure to the right, the angles A, B, C of	B
the triangle are in the ratio 5:12:13. What is
the measure of angle A?

(A)15

(B)	27	A		C
(C)	30

(D)34

(E)40

Since the angle sum of a triangle is 180°, A + B + C = 180. Forming two of the ratios yields

	A	=		5				A	=	5
	B		12					C		13
Solving the first equation for B yields			B =			12	A
						5
Solving the second equation for C yields			C =			13	A
						5

Hence, 180 = A + B + C = A + 125 A + 135 A = 6A . Therefore, 180 = 6A, or A = 30. The answer is choice (C).

288GRE Math Bible

Problem Set Q

Easy

1.At Stephen Stores, 3 pounds of cashews costs $8. What is cost in cents of a bag weighing 9 ounces?

(A)30

(B)60

(C)90

(D)120

(E)150

Medium

2.In the figure, what is the value of y if x : y = 2 : 3 ?

(A)16

(B)32

(C)48

(D)54

(E)72

y°

x°

l

m

3.In the figure, ABCD is a rectangle and points E, F, G and H are midpoints of its sides. What is the ratio of the area of the shaded region to the area of the un-shaded region in the rectangle?

(A)1 : 1

(B)1 : 2

(C)2 : 1

(D)1 : 3

(E)3 : 1

A	E	B
H		F
	G
D		C

4.If r and s are two positive numbers and r is 25% greater than s, what is the value of the ratio r/s ?

(A)0.75

(B)0.8

(C)1

(D)1.2

(E)1.25

Ratio & Proportion 289

5.The ratio of the sum of the reciprocals of x and y to the product of the reciprocals of x and y is 1 : 3. What is sum of the numbers x and y?

(A)1/3

(B)1/2

(C)1

(D)2

(E)4

6.The ratio of the number of chickens to the number of pigs to the number of horses on Richard’s farm is 33 : 17 : 21. What fraction of the animals are either pigs or horses?

(A)16/53

(B)17/54

(C)38/71

(D)25/31

(E)38/33

7.Column A

The savings from income of Marc

8.Column A

The number of cups of sugar required for the same recipe to make 30 cookies

The savings from a person’s	Column B
income is the difference between
his income and his expenditure.
The ratio of the incomes of Marc
and his boss are in the ratio 3 : 4.
Their respective expenditures ratio
is 1 : 2.	The savings from income of
	Marc’s Boss
A certain recipe requires 3/2 cups	Column B
of sugar and makes 2-dozen
cookies.	2

9.If a, b, and c are three different numbers and ax : by : cz = 1 : 2 : –3, then ax + by + cz =

(A)0

(B)1/2

(C)3

(D)6

(E)9

10.	Column A			a : b = 2 : 3.	Column B
		a + 5		a is positive.	1
		b + 5

290 GRE Math Bible

Hard

11.In Figure 1, y = 3x and z = 2x. What is the ratio p : q : r in Figure 2?

(A)1 : 2 : 3

(B)3 : 1 : 2

(C)1 : 32 : 1

(D)2 : 3 : 1

(E)3 : 2 : 1

B

60°

30°

E

z

r

x

p

A

y

C

D

q

F

Figure 1

Figure 2

Note: The figures are not drawn to scale.

12.In the figure, the ratio of the area of parallelogram ABCD to the area of rectangle AECF is 5 : 3. What is the area of the rectangle AECF ?

(A)18

(B)24

(C)25

(D)50

(E)54

	A	E	B
	5
D	F	C
		10

13.In a class, 10% of the girls have blue eyes, and 20% of the boys have blue eyes. If the ratio of girls to boys in the class is 3 : 4, then what is the fraction of the students in the class having blue eyes?

(A)11/70

(B)11/45

(C)14/45

(D)12/33

(E)23/49