Percentages

Percent means out of a hundred

15% of n is 0.15n. Turning the percent into a decimal first removes most of the difficulty.

Up 20% is × 1.2. Down 20% is × 0.8. Doing both is × 0.96, not back where you started.

(new − old)/old, then × 100. The thing you compare to goes on the bottom.