Calculate a Percentage Decrease

This activity is about calculating a percentage decrease.

We are using a calculator for this activity!

We can see percentage decreases everywhere.

A common place to see them is in shops when they have a sale!

We can calculate simple percentage decreases easily without a calculator.

For example, a pair of trainers were £60 but there was 20% off in the sale.

We can work out 20% of 60 and subtract the answer from 60.

So, 10% of 60 = £6

20% = 2 x 10% = 2 x £6 = £12

Sale price = £60 - £12 = £48  

When they get a little more tricky, like a 47% decrease, it is easier and quicker to use a multiplier and a calculator!

Recap - the multiplier is basically, the decimal!

Example

Decrease 35 ml by 15%.

Answer

To find 15% it is 15/100 = 15 ÷ 100 =  0.15 as a decimal.

If we decrease by 15% then we want the other 85%. 

Decrease means we are taking away 15% so we want 85%.    

So, we could do the whole thing in one move:

A decrease of 15% means 100% - 15%  = 85% =  0.85

 0.85 x 35 ml to get the 15% decrease.

35 ml x 0.85  = 29.75 ml

It is a bit tricky, but it will become easier with practice.

Let's try some questions.