If we are required to increase or decrease an amount by a certain percentage, the easiest way to do it is to keep in mind that the original amount represents 100% and add or subtract our new percentage to this.

So if we want to increase an amount by 15%, we can just multiply it by 115% or 115/100 or 1.15

Similarly, if we want to decrease an amount by 15%, we can just multiply it by 85% or 85/100 or 0.85.

**Example**

Let’s try some of these with our teacher:

**Exercise**

Let’s complete exercise 13C on pages 208 and 209 of the textbook:

The answers are below: