Percentage of a number using mental math
Learn how to solve percent problems in your head! Since 10% is 1/10, and it's so easy to find 1/10 of any number, we can use that to quickly calculate certain percentages of quantities in our heads. to find 20% of a number, first find 10% of it, and double that. And so on.
The basic concepts and ideas of this lesson are also explained in this video:
100% of something means all of it. 1% of something means 1/100 of it. 
To calculate a percentage of a quantity, we use the same method as when we calculate a fractional part of that quantity because percent simply means a hundredth part. Therefore, percentages are just fractions. 
How much is 1% of 200 kg? This means how much is 1/100 of 200
kg? It is simply 2 kg. You just divide by 100 to find a hundredth part! To find 1% of something (1/100 of something), divide by 100. Remember how to divide by 100 mentally: Just move the decimal point two places to the left. For example, 1% of 540 is 5.4. And 1% of 8.30 is 0.083. 
To find 10% of some quantity, divide by 10. Why does that work? 10%
is 10/100. And 10/100 is equal to 1/10. So we just find 1/10 of the
For example, 10% of 340 people is 34 people. Similarly, 10% of $2.30 is $0.23. 
1. Find 10% of these numbers.
a. 900 _______  b. 160 _______  c. 50 _______ 
2. Find 1% of these numbers.
a. 900 _______  b. 6,800 _______  c. 550 _______ 
3. If 1% of Cindy’s salary is $23, how much is her salary?
4. Ten percent of the cost of a pool is $430. How much does the pool cost?
To find 2% of a quantity, first find 1% of it, then double that. For example, let’s find 2% of $6. Since 1% of 6 is $0.06, then 2% of 6 is $0.12. 
Can you think of a way to find 20% of a number? (Hint: Start with finding 10% of the number.) ________________________________________________________________ 
7. Fill in the table. Use mental math.
percentage / number  400  60  78  8  4.1 
1% of the number  
2% of the number  
10% of the number  
20% of the number 
8. Fill in this guide for using mental math with percent:
Mental Math and Percent of a Number 




To find 20%, 30%, 40%, 60%, 70%, 80%, or 90% of a number,

10% of 120 is _______.
30 % of 120 is _______. 60 % of 120 is _______. 
9. Find percentages. Use mental math.
a. 10% of 60 kg ________ 20% of 60 kg ________ 
b. 10% of $14 ________ 30% of $14 ________ 
10. David pays a 20%
income tax on his
$2,100 salary.
How many dollars is the tax?
How much money does he have left
after paying the tax?
12. Identify the errors that these children made. Then find the correct answers.
a. Find 90% of $55.
Peter’s solution:





13. Find percentages. Use mental math.
a. 25% of 48 mi _______ 75% of 48 mi ________ 
14. Find percentages of the quantities.
a. 50% of 26 in. _______  b. 25% of 40 ft _______ 
15. Fill in the mental math method for finding 12% of $65.
10% of $65 is $________. 1% of $65 is $_________. 2% of $65 is $_________.
Now, add to get 12% of $54: $_________ + $_________ =
$_________
18. On her cell phone bill, Hannah noticed that of the
340 text messages
that she sent last month, 15% were sent at the
cheaper night rate.
How many messages did Hannah send at
night? During the day?
20. A carton contains 2 liters
of tropical juice. The juice consists of
25% mango juice, 30% pineapple juice, and the
rest is guava
juice. Find how many milliliters of each juice
went into the
2liter carton of tropical juice.
This lesson is taken from Maria Miller's book Math Mammoth Percent, and posted at www.HomeschoolMath.net with permission from the author. Copyright © Maria Miller.