Percentage calculations (2025 essential formulas): **(1) "X is what percent of Y?"** → (X / Y) × 100.

Example: Score 85 out of 100 → (85 / 100) × 100 = **85%**.

Sales $75k out of $100k target → 75%. **(2) "What is X% of Y?"** → (X / 100) × Y.

Example: 15% tip on $60 bill → (15 / 100) × 60 = **$9**. 20% down payment on $300k house → 0.20 × $300k = $60k. **(3) "X is Y% of what number?"** → X / (Y / 100).

Example: $45 is 15% of what? → $45 / 0.15 = **$300 original amount**.

Paid $2,400 tax (8% rate), income = $2,400 / 0.08 = $30,000.

**Percentage increase/decrease**: Formula: **% Change = [(New - Old) / Old] × 100**.

Positive = increase, negative = decrease.

Example: Salary $50k → $55k → ($55k - $50k) / $50k × 100 = $5k / $50k × 100 = **10% increase**.

Stock $120 → $90 → ($90 - $120) / $120 × 100 = -$30 / $120 × 100 = **-25% decrease**. **Reverse calculation**: If 10% increase = $5,000, what was original? → $5,000 / 0.10 = $50,000 original salary.

**Percentage difference (comparing two numbers)**: **% Diff = |A - B| / [(A + B) / 2] × 100**.

Example: Product A costs $80, Product B costs $100.

Difference = |$80 - $100| / [($80 + $100) / 2] × 100 = $20 / $90 × 100 = **22.2% difference**. **Common mistake**: Using % Difference vs % Change. % Change has "base" (old value), % Difference treats both equally.

**Percentage points vs percentage change**: "Interest rate rose from 5% to 6%" = **1 percentage point** increase, but **20% relative change** (1% / 5% × 100). 2025 example: Inflation fell from 8% to 6% = 2 percentage point drop, but 25% relative decrease.

Media often confuses these. **Grade calculations (2025)**: Weighted average: Homework 20% (grade 90), Midterm 30% (grade 85), Final 50% (grade 88) → (90 × 0.20) + (85 × 0.30) + (88 × 0.50) = 18 + 25.5 + 44 = **87.5% final grade**.