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How Can You Calculate Percentages?


Here you will learn about the calculation of percentages. It is often useful to know how to convert numbers to percentages and back. For example, you have probably been shopping while there has been a sale. How much will something cost if you get a 40 % discount? How do you calculate that? That is exactly what we are going to look at here.

Rule

When you want to find the percentage of a number:

percentage of a number = number a% = number a 100

percentage of a number = number a% = number a 100

Example 1

Calculate 40% of 300.

300 40% = 3 100 40 100 = 3 40 = 120

This means 40% of 300 is 120.

Example 2

Find 75% of 80.

80 75% = 80 75 100 = 80 75 100 = 8 75 10 = 8 7.5 = 60 This means 75% of 80 is 60.

What should you do if you want to add a percentage? I will show you how with two methods: The long way and the short way. The short method uses a growth factor, and you will find that it becomes your best friend. Let us try an example.

Example 3

You want to add 15% to 400.

The long way

Begin the same way you did in the examples above. First find what 15 % of 400 is:
400 15% = 400 15 100 = 4 100 15 100 = 4 15 = 60

400 15% = 400 15 100 = 4 100 15 100 = 4 15 = 60

Since you want to add 15 % to 400, you’ll have to add 60. You’ll find that
400 + 60 = 460.

The easy way: growth factor

You do both calculations in one move:
400 (1 + 15 100) = 400 1.15 = 460

Example 4

You are out shopping during a sale. You find a jacket with a discount that says 30% off. The regular price is $70. How much do you have to pay?

We’re going to use the growth factor method:

70 (1 30 100) = 70 (1 0.30) = 70 0.7 = 49

70 (1 30 100) = 70 (1 0.30) = 70 0.7 = 49

This means that a 30 % discount means you have to pay $49.

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