The three formulas
X% of Y = Y × X ÷ 100 · X as a % of Y = X ÷ Y × 100 · change = (B − A) ÷ |A| × 100
The classic mix-up
Percent change is measured against the starting value. Going 80 → 92 is a 15% increase, but 92 → 80 is a 13% decrease — same gap, different base. And a 50% drop needs a 100% gain to get back to even, which is why "percent up then percent down" traps so many people.