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Day 12 · Decimal operations and a reasonable estimateYear 7 Maths · T1 W3–4 · Day 12 lesson

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Year 7 / Mathematics / Term 1 / Weeks 03 04

Part of the full two-week lesson sequence. Check the pack guide and taught point before teaching.

Open for this lesson: Pack guide · Sources and materials · Daily routes · Worked swaps · Fresh learner checks · Aids and text routes.

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Teacher copy: This page may include teaching prompts or answer keys. Answer keys in this public library can be viewed by anyone. Give learners a clean prompt, use checks as formative evidence, and change a case locally when prior access matters.

Learner prompts

Codes: AC9M7N05, AC9M7N06. Goal: add and subtract decimals by place value, then check scale. Prepare: operation mat, decimal place cards.

  1. Launch · 2 min. Two fictional design lengths are 4.75 m and 2.60 m. Expect a total near 7–8 m, not 70 m.
  2. Model · 5 min. Align decimal places: 4.75+2.60=7.35 m. Round each input to the nearest whole metre (4.75→5, 2.60→3) for a loose estimate 5+3=8 m; this detects a decimal-shift error but does not replace the exact sum.
  3. Guide · 6 min. From a fictional 10.00 m roll, subtract 7.35 m: 10.00−7.35=2.65 m. Check 7.35+2.65=10.00. Ask why metres cannot be silently changed to centimetres.
  4. Choice · 6 min. D12-A/B/C uses a fictional art strip, data-storage readout or sports path; each shows written/inverse check and unit.
  5. Exit · 4 min. Repair 4.75+2.60=6.135: show correct 7.35 and one magnitude reason. Do not credit a close estimate as the exact answer.
  6. Note · 2 min. Save original decimal alignment and inverse. If error is in place value, use a different tenths/hundredths example next.

Optional home/extension: Use paper decimal columns; extension: express 2.65 m as 265 cm with the unit conversion stated.

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