X% of Y — multiply Y by (X/100). For example, 20% of 150 = 150 × 0.20 = 30.
X is what % of Y — divide X by Y and multiply by 100. For example, 30 is what % of 150? (30/150) × 100 = 20%.
Percentage change — subtract the original from the new value, divide by the original, and multiply by 100. A positive result is an increase; negative is a decrease. Example: from 80 to 100: ((100−80)/80) × 100 = +25%.