Grade 5 · Unit 1: Number and place value

Long multiplication

3-digit × 2-digit numbers on paper.

Garment factories make thousands of shirts each day.

Garment factory

A factory line makes 245 shirts an hour for 18 hours. How many shirts? Estimate: 250 × 20 = 5000.

Grid method

Split both numbers. Row for 10: 2000 + 400 + 50 = 2450. Row for 8: 1600 + 320 + 40 = 1960. Total 4410.

Long multiplication

Multiply 245 by 8: 1960. Multiply 245 by 10: 2450 (write the 0 first). Add the partial products: 4410.

Check

We rounded 245 up to 250 and 18 up to 20. So the real answer should be smaller than 5000. 4410 makes sense.

4-digit × 3-digit

Multiply by each part of 123: 100, 20 and 3. Add the three partial products. Estimate: 2000 × 100 = 200 000 and 2500 × 120 = 300 000, so 288 435 is sensible.

What I learnt