In earlier posts, we have learned what a percent is and how to calculate it.

Today we are going to use the rule of three to solve different types of problems related to percentages.

###### Rule of three to calculate the percentage of a number

For example, we want to calculate 30% of 360.

30% means 30 for each 100. So the approach would be: if I have 30 from 100, I have *X* from 360:

**100 —— 30**

**360 —— X**

*X* = (30 x 360) / 100

*X* = 108

So, 30% of 360 is 108.

###### Rule of three to calculate a quantity knowing the percentage of it

For example, we know that 25% of a quantity is 49. What is the quantity?

If 25% is 49 then the 100%, which we do not know, will be *X* :

**25 —— 49**

**100 —— X**

*X* = (49 x 100) / 25

*X* = 196

The quantity we are looking for is 196.

###### Rule of three to calculate the percentage represented as a quantity of another

What percentage of 250 does 50 represent?

250 is the 100% and 50 is the percentage that we do not know, *X* :

**250 —— 100**

**50 —— X
**

*X* = (100 x 50) / 250

*X* = 20

50 is 20% of 250.

**Rule of three to calculate the percentage of an unknown quantity knowing another percentage of the quantity**

We know that 40% of a quantity is 78, how much would 60% be of the same quantity?

The 40% is 78 and we want to calculate 60%, which will be *X *:

**40 —— 78**

**60 —— X**

*X* = (78 x 60) / 40

*X* = 117

So 60% of this quantity is 117.

Today we have learned to solve different problems related to percentages using the same tool: the rule of three. To learn more about this and other contents from primary mathematics, check out Smartick and try it for free.

Learn More:

- Learn the Basic Concepts of the Rule of 3
- Direct and Inverse Rule of 3 Problems
- Percentage: What Is It and How do We Calculate It?
- Singapore Bar Model and Percentages
- How to Calculate the Least Common Multiple Using a 100 Square