Math lesson plan · 9th grade (Year 10 in England) · 30 minutes
Compound interest and depreciation
Decimals and percentages
CCSS HSF.LE.A.1.CCCSS HSA.SSE.B.3.CEngland: GCSE Foundation RatioEngland: GCSE Higher Ratio
Objective and success criteria
Students will: Compound interest and depreciation.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Percentage increase and decrease; Whole-number exponents
Leads to: Exponential growth and decay; Compare compounding periods and continuous interest
Materials: Place value chart with tenths and hundredths, Hundred squares, Number lines, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 4 min
- Quick retrieval from earlier learning (Percentage increase and decrease; Whole-number exponents). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- A bike costs $40. In a sale it is 15% off. What is the sale price? (Type the number only.) → 34Prompt: Find 15% of 40 first.
- Evaluate 6⁴. → 1296Prompt: Write it out: 6 × 6 × 6 × 6
- Increase 380 by 8%. → 410.4Prompt: Find 8% of 380 and add it on.
Hook · 3 min
- Show two prices or race times with decimals and ask which is bigger, then why.
- Share the objective: Compound interest and depreciation. Read the success criteria together.
I do: model it · 7 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: $4800 is saved at 4% compound interest per year. How much is in the account after 3 years? Round to 2 decimal places (type the number only).
- Step 1: 4800 × 1.04³ = 5399.3472
- Step 2: ≈ 5399.35
- Answer: 5399.35
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 6 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- $4200 is saved at 5% compound interest per year. How much is in the account after 2 years? Round to 2 decimal places (type the number only). → 4630.5Prompt: Each year the amount is multiplied by 1.05.
- A car worth $17000 loses 15% of its value each year. What is it worth after 4 years? Round to the nearest whole number (type the number only). → 8874Prompt: Each year the value is multiplied by 0.85.
You do: independent practice · 7 min
- Students work on the student sheet. Circulate and check the first two answers of every student early.
- Students who finish: write a question of their own for a partner, with the answer.
The 6 questions are on the student sheet below, with answers in the key.
Exit ticket · 3 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- $3500 is saved at 10% compound interest per year. How much is in the account after 3 years? Round to 2 decimal places (type the number only). → 4658.5Prompt: Each year the amount is multiplied by 1.1.
- A car worth $9000 loses 25% of its value each year. What is it worth after 2 years? Round to the nearest whole number (type the number only). → 5063Prompt: Each year the value is multiplied by 0.75.
Watch out for
- Thinking 0.45 is bigger than 0.5 because 45 is bigger than 5.
- Moving the digits the wrong way when multiplying or dividing by 10.
Support and stretch
Support
- Review first: Percentage increase and decrease.
- Use concrete materials: place value chart with tenths and hundredths.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Move on to: Exponential growth and decay; Compare compounding periods and continuous interest.
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/math/grade-9.
Free lesson plan from eurekami.com/lesson-plan-builder