How to Calculate Percentages: 3 Methods Everyone Should Know

The Three Percentage Questions

Almost every percentage problem is one of these three:

1. What is X% of Y? (e.g., "What is 15% of 200?")
2. What's the percentage change? (e.g., "Price went from £50 to £65")
3. A is what % of B? (e.g., "30 is what percent of 150?")

Method 1: Finding X% of Y

Answer = (X ÷ 100) × Y

Example: 15% of 200 = (15 ÷ 100) × 200 = 0.15 × 200 = 30

Quick trick: 10% is just moving the decimal. 15% = 10% + 5% (half of 10%).

Method 2: Percentage Change

Change = ((New - Old) ÷ Old) × 100

Example: Price from £50 to £65 = ((65-50) ÷ 50) × 100 = +30%

If the result is negative, it's a decrease.

Method 3: A is What % of B

Percentage = (A ÷ B) × 100

Example: 30 is what % of 150? = (30 ÷ 150) × 100 = 20%

Real-World Uses

• Discounts: "30% off £80" = 0.30 × 80 = £24 off → pay £56
• Tips: "20% tip on £45" = 0.20 × 45 = £9 tip
• Grades: "Got 42 out of 50" = (42÷50) × 100 = 84%
• Salary raise: "5% raise on £35,000" = 0.05 × 35,000 = £1,750 more