The three formulas
X% of Y = Y × X ÷ 100 (15% of 240 is 36). X as a percentage of Y = X ÷ Y × 100 (36 is 15% of 240). Percentage change = (new − old) ÷ old × 100 — note the divisor is always the old value, which is why a 50% fall needs a 100% rise to get back to even, the asymmetry that catches investors out. And a GST-flavoured reminder from our other calculators: adding 10% and then taking 10% off does not return you to the start — that's the divide-by-11 trap in another costume.