Decimals and Percentage

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PSAT Math › Decimals and Percentage

Questions 1 - 10
1

If 20% of x is equal to 160% of y, then y is what percent of x?

800

12.5

125

1.25

8

Explanation

We are told that 20% of x is equal to 160% of y. We need to write expressions for 20% of x and 160% of y and set them equal to one another.

In order to find an expression for 20% of x, we can write 20% as a decimal and multiply it by x. Since 20% = 0.20, we can write 20% of x as 0.2x.

Similarly, we can write 160% as 1.60y.

Now, we set these expressions equal to one another.

0.2x = 1.60y.

Since the question asks us to find y as a percentage of x, we need to solve for y in terms of x. Let's divide both sides of the equation by 1.60.

0.125x = y.

Therfore, y is equal to 0.125x, which is the same as 12.5% of x, since 12.5% expressed as a decimal is 0.125.

The answer is 12.5.

2

Express .789 as a percentage.

78.9%

7.89%

0.789%

789%

Explanation

Move the decimal two places to the left to convert a decimal to a percentage. Therefore, .789 is equivalent to 78.9%.

3

If 20% of x is equal to 160% of y, then y is what percent of x?

800

12.5

125

1.25

8

Explanation

We are told that 20% of x is equal to 160% of y. We need to write expressions for 20% of x and 160% of y and set them equal to one another.

In order to find an expression for 20% of x, we can write 20% as a decimal and multiply it by x. Since 20% = 0.20, we can write 20% of x as 0.2x.

Similarly, we can write 160% as 1.60y.

Now, we set these expressions equal to one another.

0.2x = 1.60y.

Since the question asks us to find y as a percentage of x, we need to solve for y in terms of x. Let's divide both sides of the equation by 1.60.

0.125x = y.

Therfore, y is equal to 0.125x, which is the same as 12.5% of x, since 12.5% expressed as a decimal is 0.125.

The answer is 12.5.

4

Express .789 as a percentage.

78.9%

7.89%

0.789%

789%

Explanation

Move the decimal two places to the left to convert a decimal to a percentage. Therefore, .789 is equivalent to 78.9%.

5

To get on the ballot for the student body president at Harding High School, a student must turn in a petition with the signatures of 8% of the students from each of the four classes - freshman, sophomore, junior, and senior. There are 342 freshmen, 312 sophomores, 270 juniors, and 268 seniors enrolled at McKinley.

Tom has a petition with the signatures of 25 students from each of the four classes. Can he get on the ballot with the signatures he has, and if not, why not?

Tom cannot get on the ballot yet, because he needs more signatures from freshmen.

Tom cannot get on the ballot yet, because he needs more signatures from sophomores.

Tom cannot get on the ballot yet, because he needs more signatures from members of all four classes.

Tom cannot get on the ballot yet, because he needs more signatures from both freshmen and sophomores.

Tom has the signatures he needs to get on the ballot.

Explanation

In order to answer the question, we must find out the percent of each class that has signed Tom's petition, and compare it to 8%.

Freshmen: have signed.

Sophomores: have signed.

Juniors: have signed.

Seniors: have signed.

Tom has the necessary signatures from members of the top three classes, but he cannot get on the ballot yet because he has not gathered enough signatures from freshmen.

6

55 and 1/2% of 23 is about what?

49

13

11

155

2

Explanation

55 and 1/2% can be written as a decimal: 0.555. To see what number is about 55.5% of 23, multiply 0.555 by 23. Answer: 12.765 or about 13.

Another route is to say that 55.5% is about half of 23. Half of 23 is 11.5. Since 55.5% is greater than 50%, 13 is the logical choice instead of 11.

7

Let x and y be numbers such that x and y are both nonzero, and x > y. If half of x is equal to thirty percent of the positive difference between x and y, then what is the ratio of x to y?

3/2

2/3

–3/2

–2/3

–1

Explanation

We need to find expressions for fifty percent of x and for thirty percent of the positive difference between x and y. Then, we can set these two expressions equal to each other and determine the ratio of x to y.

Fifty percent of x is equal to one-half of x, which is the same as multiplying x by 0.50.

50% of x = 0.5_x_

Thirty percent of the positive difference between x and y means that we need to multiply the positive difference between x and y by thirty percent. Because x > y, the positive difference between x and y is equal to xy. We then need to take thirty percent of the quantity xy. Remember that to convert from a percent to a decimal, we move the decimal two spaces to the left. Therefore, 30% = 0.30. We can now multiply this by (x – y).

30% of xy = 0.30(xy)

Now, we set the two expressions equal to one another.

0.5_x_ = 0.30(xy)

Distribute the right side.

0.5_x_ = 0.3_x_ – 0.3_y_

The ratio of x to y is represent by x/y. Thus, we want to group the x and y terms on opposite sides of the equations, and then divide both sides by y.

0.5_x_ = 0.3_x_ – 0.3_y_

Subtract 0.3_x_ from both sides.

0.2_x_ = –0.3_y_

Divide both sides by 0.2

x = (–0.3/0.2)y

Divide both sides by y to find x/y.

x/y = (–0.3/0.2) = –1.5.

Because the answers are in fractions, we want to rewrite –1.5 as a fraction. We can write –1.5 as –1.5/1 and then mutiply the top and bottom by 2.

(–1.5/1)(2/2) = –3/2

The answer is –3/2

8

55 and 1/2% of 23 is about what?

49

13

11

155

2

Explanation

55 and 1/2% can be written as a decimal: 0.555. To see what number is about 55.5% of 23, multiply 0.555 by 23. Answer: 12.765 or about 13.

Another route is to say that 55.5% is about half of 23. Half of 23 is 11.5. Since 55.5% is greater than 50%, 13 is the logical choice instead of 11.

9

To get on the ballot for the student body president at Harding High School, a student must turn in a petition with the signatures of 8% of the students from each of the four classes - freshman, sophomore, junior, and senior. There are 342 freshmen, 312 sophomores, 270 juniors, and 268 seniors enrolled at McKinley.

Tom has a petition with the signatures of 25 students from each of the four classes. Can he get on the ballot with the signatures he has, and if not, why not?

Tom cannot get on the ballot yet, because he needs more signatures from freshmen.

Tom cannot get on the ballot yet, because he needs more signatures from sophomores.

Tom cannot get on the ballot yet, because he needs more signatures from members of all four classes.

Tom cannot get on the ballot yet, because he needs more signatures from both freshmen and sophomores.

Tom has the signatures he needs to get on the ballot.

Explanation

In order to answer the question, we must find out the percent of each class that has signed Tom's petition, and compare it to 8%.

Freshmen: have signed.

Sophomores: have signed.

Juniors: have signed.

Seniors: have signed.

Tom has the necessary signatures from members of the top three classes, but he cannot get on the ballot yet because he has not gathered enough signatures from freshmen.

10

Let x and y be numbers such that x and y are both nonzero, and x > y. If half of x is equal to thirty percent of the positive difference between x and y, then what is the ratio of x to y?

3/2

2/3

–3/2

–2/3

–1

Explanation

We need to find expressions for fifty percent of x and for thirty percent of the positive difference between x and y. Then, we can set these two expressions equal to each other and determine the ratio of x to y.

Fifty percent of x is equal to one-half of x, which is the same as multiplying x by 0.50.

50% of x = 0.5_x_

Thirty percent of the positive difference between x and y means that we need to multiply the positive difference between x and y by thirty percent. Because x > y, the positive difference between x and y is equal to xy. We then need to take thirty percent of the quantity xy. Remember that to convert from a percent to a decimal, we move the decimal two spaces to the left. Therefore, 30% = 0.30. We can now multiply this by (x – y).

30% of xy = 0.30(xy)

Now, we set the two expressions equal to one another.

0.5_x_ = 0.30(xy)

Distribute the right side.

0.5_x_ = 0.3_x_ – 0.3_y_

The ratio of x to y is represent by x/y. Thus, we want to group the x and y terms on opposite sides of the equations, and then divide both sides by y.

0.5_x_ = 0.3_x_ – 0.3_y_

Subtract 0.3_x_ from both sides.

0.2_x_ = –0.3_y_

Divide both sides by 0.2

x = (–0.3/0.2)y

Divide both sides by y to find x/y.

x/y = (–0.3/0.2) = –1.5.

Because the answers are in fractions, we want to rewrite –1.5 as a fraction. We can write –1.5 as –1.5/1 and then mutiply the top and bottom by 2.

(–1.5/1)(2/2) = –3/2

The answer is –3/2

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