If a quantity increases by $20\%$ and then decreases by $20\%$, its final value is the same as its initial value.

To find a quantity that is $25\%$ less than $X$, one can calculate $X-0.25X$ or $0.75X$.

If quantity $A$ is $50\%$ greater than quantity $B$, then quantity $B$ is $50\%$ less than quantity $A$.

The percentage increase from $100$ to $120$ is $20\%$, and the percentage decrease from $120$ to $100$ is also $20\%$.