Calculate Reverse Percentages

Have you ever noticed how the 'sale' sign almost has a magnetic effect on shoppers?

 

You may have found yourself drawn to such an aisle in a store and found yourself picking up items you really don't need because the helpful little sign informs you that it is '20% off'.

 

Does this sound familiar?

In this situation, it'd be really helpful to know what the original cost of the item was as this would allow us to work out if it's worth splashing the cash.

 

Let's take the following example:

 

In a sale, it is 40% off all prices.

You pay £120 for a TV.

How much was the TV before the sale?

1) We know that the price listed must, therefore, be 60% of the total cost.
100% (Original Cost) - 40% (Sale amount) = 60% (This is how much of the original price the TV is currently worth)

2) We should now set this up as a ratio, using the following structure:
£ : %

When we input the numbers here we get:

£120 : 60%

3) We have our ratio set up, we now need to find out how much equalled 100% or the original cost. 

The easiest thing to do is divide both sides by the percentage to find 1%, then multiply both sides by 100.

 

£120 : 60% ÷ both sides by 60.  

This gives us the ratio: 2:1 

 

Now x both sides by 100. This will give us 100% of the original cost of the TV.

£200 : 100%

 

So, there we have our answer.  Seems, like that was quite a good bargain after all!

 

In this activity, that's exactly what we'll be doing.  We'll be recapping how to reverse the percentages, so that we can work out if these 'bargains' are really worth it.