## Mental math with percents

There are some simple tricks for doing percents problems using mental math. In particular, this lesson will show you how to do mental math for...

• 50%
• 25%
• 10%
• 20%
• 1%

## 50%

Since $50\mbox{%}=\frac{1}{2}$, then the shortcut for finding 50% of a number is to divide by 2.

Example: What is 50% of 850?

Solution: 850 ÷ 2 = 425. So, 50% of 850 is 425.

## 25%

Since $25\mbox{%}=\frac{1}{4}$, then the shortcut for finding 25% of a number is to divide by 4.

Example: What is 25% of 68?

Solution: 68 ÷ 4 = 17. So, 25% of 68 is 17.

## 10%

Since $10\mbox{%}=\frac{1}{10}$, then the shortcut for finding 10% of a number is to divide by 10. NOTE: A trick for dividing by 10 is to move the decimal one place to the left.

Example: What is 10% of 192?

Solution: 192 ÷ 10 = 19.2. So, 10% of 192 is 19.2.

## 20%

Since 20% is twice as much as 10%, the shortcut for finding 20% is to find 10% and then double it.

Example: What is 20% of 41?

Solution: 10% of 41 is 4.1, so 20% is 8.2.

## 1%

Since $1\mbox{%}=\frac{1}{100}$, then the shortcut for finding 1% of a number is to divide by 100. NOTE: A trick for dividing by 100 is to move the decimal two places to the left.

Example: What is 1% of 720?

Solution: 720 ÷ 100 = 7.2, so 1% of 720 is 7.2.

Look at these shopping examples.

 50% 25% 10% 20% 1% Divide by 2 Divide by 4 Divide by 10 Or Move decimal to the left ONE space Divide by 5 Or Find 10% and then DOUBLE it Divide by 100 Or Move decimal to the left TWO spaces $80 80 ÷ 2 =$40 80 ÷ 4 = $20 80 ÷ 10 =$8 80 ÷ 10 = 8 8 x 2 = $16 80 ÷ 100 =$0.80 $250 250 ÷ 2 =$125 250 ÷ 4 = $62.50 250 ÷ 10 =$25 250 ÷ 10 = 25 25 x 2 = $50 250 ÷ 100 =$2.50 $4 4 ÷ 2 =$2 4 ÷ 4 = $1 4 ÷ 10 =$0.40 4 ÷ 10 = 0.40 0.40 x 2 = $0.80 4 ÷ 100 =$0.04

# Self-Check

Question 1

What is 25% of 32?

Question 2

How much is 10% of 549?

Question 3

Find 20% of 35.