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