Study guides › Level 6 · Master

Percent change

Level 6 · Master · for questions like “A price goes from $80 to $100. What is the percent change?”

💡 The idea

Percent change says how big a change is compared with where you started.

percent change = change ÷ original × 100%

🪜 The steps

1. Find the change: new − old. Note whether it went up (an increase) or down (a decrease).
2. Divide by the original amount, the old one, never the new one.
3. Turn it into a percent: × 100.

✏️ Worked examples

1. $80 → $100. The change is $20 up. 20 ÷ 80 = 0.25, which is a 25% increase.

2. $200 → $150. The change is $50 down. 50 ÷ 200 = 0.25, which is a 25% decrease.

⚠️ Watch out

• Divide by the starting amount. 20 ÷ 100 gives 20%, which is wrong for example 1.
• Up 25%, then down 25%, doesn't bring you back: $80 → $100 → $75.

🎯 Try these

1. $40 → $50
2. $120 → $90
3. $250 → $300

Answers: 1) 25% increase 2) 25% decrease 3) 20% increase

This guide is also in the 10MathQ app: tap “📖 Study guide” after a question.