What is 15% of 80? 12 is what percent of 80? Both directions, plus percent change - with the arithmetic shown so you can check it yourself.
Almost every percentage problem is one of three shapes. "What is 15% of 80?" multiplies: 80 × 0.15 = 12. "12 is what percent of 80?" divides: 12 ÷ 80 = 0.15 = 15%. "What's the change from 80 to 92?" compares: (92 − 80) ÷ 80 = 15% increase. The toggle above switches between them, and the answer always shows the arithmetic so you can check it - or reuse it without the tool next time.
Percent change is measured against the starting value, which is why going from 80 to 100 is a 25% increase, but going back from 100 to 80 is only a 20% decrease. Same 20-point gap, different percentages - because the base you divide by changed. This tool always uses the "from" value as the base and says so in the result.
Working out a tip or splitting a bill? The unit price comparator handles which-deal-is-cheaper. Comparing grades? The grade calculator works in weighted percentages.