You need to be familiar with percentage increase and decrease calculations, which find amounts after a percentage change. 'Reverse percentages' is the term often used to find the amount before a percentage change, when you only know the amount after. It is therefore like 'going backwards' on the percentage change.
A good way to perform percentage increase and decrease calculations is using the multiplier. This is explained in the first video. The second and third videos then explain how to apply this to reverse percentages.