Step by step
The question
Question: 21 is 70% of what number?
70% of ? = 21
We write it like this: 70% of the number is 21.
21 is a part.
The whole number is what we want.
? = the whole number
Careful!
Many people work out 70% of 21.
But 21 is not the whole.
It is only 70% of it.
Careful: not 70% of 21 ✗
1 Write the percent as a fraction
Percent means out of 100.
So 70% is 70/100.
70% = 70100
70 and 100 can both be divided by 10.
So 70% is 7/10: 7 of 10 equal parts.
= 70 ÷ 10100 ÷ 10 = 710
2 Draw the whole
3 Find one part
7 parts make 21.
So one part is 21 divided by 7.
That is 3.
1 part = 21 ÷ 7 = 3
Every part is 3.
4 Find the whole
The whole is 10 parts.
10 times 3 is 30.
10 parts = 10 × 3 = 30
So 21 is 70% of 30.
5 Check
Let's check: is 70% of 30 really 21?
70% is 7/10.
Of means times.
30 divided by 10 is 3.
7 times 3 is 21.
70% of 30 = 710 × 30 = 7 × 3 = 21 (30 ÷ 10 = 3)
We get 21.
So it is right!
Let's look back.
First: write the percent as a fraction.
Then: draw the whole.
Then: find one part.
Then: find the whole.
Last: check.
Answer
10 parts = 10 × 3 = 30