X% of Y = (X/100) × Y
Example: 15% of 200 = (15/100) × 200 = 30
X is Z% of Y → Z = (X/Y) × 100
Example: 25 is what % of 200? → (25/200) × 100 = 12.5%
Percentage Change = ((New - Old)/Old) × 100
Example: Change from 80 to 100 = ((100-80)/80) × 100 = +25%