The pair 80 and 100 demonstrates why percentage change is directional. Moving forward uses 80 as the base; moving back uses 100. The numerical difference stays 20, but the relative percentage changes.
Calculate the increase from the original 80
The change is 100 − 80 = 20. Relative to the original: 20 ÷ 80 × 100 = 25%. Equivalently, multiplying 80 by 1.25 produces 100.
Reverse the 20% decrease by division
A 20% decrease leaves a multiplier of 0.8. If the value after that decrease is 100, the original was 100 ÷ 0.8 = 125. Adding 20% to 100 gives 120 and does not undo the decrease because it applies the rate to a smaller base.