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## Rule of Three for Calculating Percentages

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 :

40 —— 78

60 —— X

X = (78 x 60) / 40

X = 117

So 60% of this quantity is 117.