SubjectNest · Year 3 maths · Make it fit, then check
Day 27 · Check a total before trusting it
Would that answer make sense? Show one check.
Usual communication and wanted help are welcome. Choose one route; rest or finish when useful.
Full session and all three routes · PDF · Full readable session · HTML
Grown-up: full preparation, teaching and three choices
Day 27 · Check a total before trusting it
Would that answer make sense? Show one check.
Useful purpose: Estimate an additive result, solve with place-value strategies and use context to catch a large error.
1 · Find the starting point
Check regrouping with a new two-digit sum and reading three-digit quantities. Keep calculator-free strategy evidence distinct from optional later digital checking.
2 · Prepare a suitable invitation
Supply paper/counters/number-line choice and MODEL 218 paper badges plus 167. These are fictional planning quantities, not real event attendance or collected data. No fast-answer contest.
3 · Brief worked model
218 is near 200 and 167 near 200, so about 400 is useful. Exact:218 + 167 = 385. I combine 200 + 100 = 300, 10 + 60 = 70, 8 + 7 = 15;300 + 70 + 15 = 385. A reported 38 is far too small for two positive hundreds. An estimate can catch that error but does not prove every digit of 385.
4 · Guided new try
Try a new sum with different values. Invite a partition, number line or mental strategy. Ask which parts combine and whether a different correct route is possible.
5 · Own attempt
Estimate first, calculate, then explain a check in the situation. If using a digital checker, save the child's prior reasoning separately; a displayed answer alone is not a strategy explanation.
6 · Observe and respond
Look for sensible scale, complete place-value parts and a contextual unit. Accept sound alternative strategies; note adult computation help explicitly.
Specific next teaching
If a place is lost, represent another addition with grouped tens/ones. If correct but unexplained, ask the child to connect each part to the objects in a new context; if secure, check by subtraction.
Pick a route
1 · Badge planner
Could we have enough?
MODEL 218 + 167 badges, stock 400. Estimate about 400, calculate 385, then explain 15 spare. Use a drawing, gesture with group labels, jottings or usual communication.
Observe: 385 total/15 spare and sensible estimate.
Next teaching: Change both batches for a new stock decision.
Evidence boundary: Fictional planning calculation.
2 · Catch the missing place
Which answer needs checking?
MODEL 274 + 189. Compare answers 46 and 463 using a rough 300 + 200 estimate, then solve 463 and identify why 46 loses the scale of the quantities.
Observe: Estimated scale, exact 463 and explanation rather than guessing the longer numeral.
Next teaching: Use a new answer with a subtle digit error requiring an inverse check.
Evidence boundary: Supplied error analysis.
3 · Build your own help ticket
Write a total someone could use.
Choose two safe known fictional batches between 100 and 300, state the unit and a practical decision. Estimate, solve and ask a helper to check your explanation without silently rewriting it.
Observe: Complete self-chosen model/question and linked quantities.
Next teaching: Ask what changes if stock is smaller than the exact total.
Evidence boundary: Original modelling proposal; no actual collection implied.