If a shop sells a bucket at £16 after a 15% discount, how much would the bucket have been before the discount?

Define the original cost as 100%.

So £16=85% (100-15) of the original cost.

Find 1% of the original cost by dividing both sides by 85. 

16/85=85/85%

To find the orginal cost, 100%, multiply both sides by 100.

16/85 * 100 Is the answer. 

This is equivilant to multiplying by 100/85 or dividing by 85/100 (as 1/85/100 = 85/100). Once understanding is reached this can be used as a shortcut as follows.

If there has been a 15% discount, divide by 1-0.15. If there has been an increase of 25%, divide by 1+0.25.