Math lesson plan · 7th grade (Year 8 in England) · 45 minutes
Percent error of an estimate or measurement
Decimals and percentages
CCSS 7.RP.A.3England: KS3 Number
Objective and success criteria
Students will: Percent error of an estimate or measurement.
- 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: Find the percentage change
Leads to: the next skill in this area.
Materials: Place value chart with tenths and hundredths, Hundred squares, Number lines, Mini whiteboards and pens, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Find the percentage change). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- A club's membership rose from 40 to 42. What is the percentage increase? → 5Prompt: Find the increase first.
- A phone's price dropped from $540 to $459. What is the percentage decrease? → 15Prompt: Find the decrease.
- A runner's 5 km time changed from 70 minutes to 47 minutes. What is the percentage decrease? Round to 1 decimal place. → 32.9Prompt: Change ÷ original × 100.
Hook · 5 min
- Show two prices or race times with decimals and ask which is bigger, then why.
- Share the objective: Percent error of an estimate or measurement. Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: Maya estimated there were 279 people at a concert. There were actually 310. What is the percent error?
- Step 1: |279 − 310| = 31
- Step 2: 31 ÷ 310 × 100 = 10%
- Answer: 10
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- Elena estimated there were 300 people at a concert. There were actually 400. What is the percent error? → 25Prompt: Percent error = |estimate − actual| ÷ actual × 100.
- A ruler measurement gave 24 cm for an object that is really 28 cm long. What is the percent error? Round to 1 decimal place. → 14.3Prompt: Find the size of the error.
You do: independent practice · 10 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 8 questions are on the student sheet below, with answers in the key.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- Lucia estimated there were 224 people at a concert. There were actually 280. What is the percent error? → 20Prompt: Percent error = |estimate − actual| ÷ actual × 100.
- A ruler measurement gave 25 cm for an object that is really 30 cm long. What is the percent error? Round to 1 decimal place. → 16.7Prompt: Find the size of the error.
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: Find the percentage change.
- 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
- 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-7.
Free lesson plan from eurekami.com/lesson-plan-builder