Reverse percentages ask you to find the amount before a percentage change. First identify the original amount, the final amount, and whether the change was an increase or a decrease.

The forward rule is:

$$F=O\times m$$

where $m$ is the multiplier. To work backwards, use the inverse formula:

$$O=\frac{F}{m}$$

For an increase, the multiplier is more than $1$, like $1.10$ for a $10\%$ increase. For a decrease, the multiplier is less than $1$, like $0.80$ for a $20\%$ decrease.