Percentages, demystified
Percent literally means 'per hundred', so any percentage is just a fraction with denominator 100. 15% = 15/100 = 0.15. Once you internalise that, every percentage problem reduces to ordinary multiplication or division.
The three building blocks are: (1) 'X% of Y' which is X/100 × Y, (2) 'X is what % of Y' which is (X/Y) × 100, and (3) '% change from A to B' which is ((B−A)/A) × 100. The 9 modes in this calculator are all combinations of these three primitives.
Common gotcha: a 20% increase followed by a 20% decrease does NOT return you to the original — it returns 96%. The calculator's increase/decrease modes make this easy to verify.