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...
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.
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.
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.
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.
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 
Question 1 What is 25% of 32? 
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Question 2 How much is 10% of 549?

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Question 3 Find 20% of 35.

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