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?

[show answer]


Question 2

How much is 10% of 549?

 

[show answer]


Question 3

Find 20% of 35.

 

[show answer]