Estimate before you multiply

Grade 6 mathematics · 15 minutes · Decimal operations

Use estimation and place value to check the reasonableness of decimal products.

Classroom activity

Apples cost $2.40 per kilogram. A bag weighs 1.5 kilograms.

1. Before calculating, decide whether the price should be below $2.40, between $2.40 and $4.80, or above $4.80. Explain.
2. Find the exact price by splitting 1.5 kg into 1 kg and 0.5 kg.
3. Compare your method with 24 × 15 = 360. Explain the decimal placement.
4. A second bag weighs 0.75 kg. Predict whether it costs more or less than $2.40, then find its price.

Compare estimates first so the exact calculation has a reasonableness check.

Check understanding

Ribbon costs $3.20 per metre. Find the cost of 0.5 metre and explain why the product is smaller than 3.20.

Teacher guide and answers

The first bag costs $3.60, within the estimated interval. The second costs $1.80. In 2.4 × 1.5, the integer factors were each scaled by 10, so divide 360 by 100. The check is $1.60, half the price for one metre. Support with money and halves; extend with 1.25 kg at the original rate.

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Grade 6 topic reference. Confirm your adopted standards; topic guidance does not certify alignment.