# 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.

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