The ideas in this lesson are taken from Math Mammoth Percent book.
Some concepts and ideas of this lesson are also explained in this video:
Percentage of a Number
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 _______  d.f. [available in the book] 
2. Find 1% of these numbers.
a. 900 _______  b. 6,800 _______  c. 550 _______  d.f. [available in the book] 
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?
5. [available in the book]
6. [available in the book]
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 ________ 
c. f.[available in the book] 
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?
11. [available in the book]
12. Identify the errors that these children made. Then find the correct answers.
a. Find 90% of $55.
Peter’s solution:

b. [available in the book] 




13. Find percentages. Use mental math.
a. 25% of 48 mi _______ 75% of 48 mi ________ 
b. [available in the book]  c. [available in the book] 
14. Find percentages of the quantities.
a. 50% of 26 in. _______  b. 25% of 40 ft _______  c.f. [available in the book] 
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: $_________ + $_________ =
$_________
16. [available in the book]
17. [available in the book]
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?
19. [available in the book]
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.
21. [available in the book]
This lesson is taken from my book Math Mammoth Percent.
Math Mammoth Percent
A selfteaching worktext for 5th7th grade that covers percent, percentage of a number, discounts, sales tax, percent of change, and comparisons with percent.
Download $3.20 or Pay What You Want!
Also available as a printed copy.