# PSAT Math : Percentage

## Example Questions

### Example Question #1 : How To Find Percentage Equivalent To A Decimal

Express .789 as a percentage.

78.9%

7.89%

0.789%

789%

78.9%

Explanation:

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

### Example Question #1 : How To Find Decimal Equivalent To A Percentage

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

49

2

11

155

13

13

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.

### Example Question #42 : Percentage

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?

–2/3

3/2

–3/2

2/3

–1

–3/2

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 = 0.5x

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 x – y. We then need to take thirty percent of the quantity x – y. 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 x – y = 0.30(x – y)

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

0.5x = 0.30(x – y)

Distribute the right side.

0.5x = 0.3x – 0.3y

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.

0.5x = 0.3x – 0.3y

Subtract 0.3x from both sides.

0.2x = –0.3y

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

### Example Question #41 : Percentage

If  of  is equal to  of , and  of  is equal to  of , then what percent of  is ?

100

133

25

75

125

75

Explanation:

We are told that 50% of x is equal to 25% of y. We need to represent these two pieces of information as algebraic expressions. We can convert 50% and 25% to decimals by moving the decimals two places to the left. Thus, 50% = 0.50, and 25% = 0.25. To find 50% of x, we multiply x by 0.50. In other words, 50% of x = 0.50x. Likewise, 25% of y = 0.25y. We now set 0.50x and 0.25y equal to one another.

0.50x = 0.25y

Let's divide both sides by 0.25 to get rid of decimals.

2x = y

Next, we are told that 40% of y is equal to 60% of z. We will represent 40% and 60% as 0.40 and 0.60, respectively. Thus, we can write the following equation:

0.40y = 0.60z

Ultimately, we are asked to find x as a percentage of z. This means we want to find an equation with x and z, but not y. If we solve for y in the second equation, and then substitute this value into the first, we can eliminate y.

Let's take the equation 0.40y = 0.60z and divide both sides by 0.40.

y = 1.5z

Now, we can take 1.5z and substitute this for y in the first equation.

2x = 1.5z

In order to find x as a percent of z, we must solve for x in terms of z. This means we must divide both sides of the equation by 2.

x = 0.75z

x is 0.75 times z. We can represent 0.75 as 75%, because in order to convert from a decimal to a percent, we need to move the decimal two spaces to the right. Therefore, if x = 0.75z, then x = 75% of z.

The answer is 75.

### Example Question #3 : Decimals And Percentage

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 members of all four classes.

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

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 both freshmen and sophomores.

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 freshmen.

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.

### Example Question #1 : How To Find A Fraction From A Percentage

Write as a fraction: 22%

4/7

4/9

11/100

11/50

2/3

11/50

Explanation:

22% = 22/100

Divide everything by 2:

22/100 = 11/50

11 is a prime number, so this is as reduced as this fraction can get.

### Example Question #1 : Percentage

When y is decreased by ten percent, the result is equal to fifteen percent of x. Assuming both x and y are nonzero, what is the ratio of x to y?

18

1/3

3

6

1/6

6

Explanation:

The problem states that decreasing y by ten percent gives us the same thing as taking fifteen percent of x. We need to find an expression for decreasing y by ten percent, and an expression for fifteen percent of x, and then set these two things equal.

If we were to decrease y by ten percent, we would be left with ninety percent of y (because the percentages must add to one hundred percent). We could write ninety percent of y as 0.90y = (90/100)y = (9/10)y. Remember, when converting from a percent to a decimal, we need to move the decimal two places to the left.

Similarly, we can write 15% of x as 0.15x = (15/100)x = (3/20)x.

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

(9/10)y = (3/20)x

Multiply both sides by 20 to eliminate fractions.

18y = 3x

The question asks us to find the ratio of x to y, which is equal to x/y. Thus, we must rearrange the equation above until we have x/y by itself on one side.

18y = 3x

Divide both sides by 3.

6y = x

Divide both sides by y.

6 = x/y

Thus, the ratio of x to y is 6.

The answer is 6.

### Example Question #1 : How To Find A Fraction From A Percentage

Write 7.5% as a fraction.

Explanation:

First convert the percentage to a decimal:

7.5% = .075

Then turn this into a fraction:

.075 = 75/1000

Simplify by dividing the numerator and denominator by 25:

75/1000 = 3/40

### Example Question #1 : Fractions And Percentage

25% of 64 is equal to 5% of what number?

320

108

94

112

90

320

Explanation:

25% of 64 is 16 (you can find this with a calculator by 0.25 * 64). Divide 16 by 0.05 (or 1/20) to get the value of the number 16 is 5% of. (Or mental math of 16 * 20)

### Example Question #61 : Percentage

Convert 62% into simplified fraction form.

Explanation:

To convert a percent into a fraction, simply divide by 100 and reduce. In our case, 62 and 100 have the common factor of 2, so solving and simplification are as follows: