Grade 5 · Unit 1: Number and place value

Mental multiplication

Double, halve, split and use facts.

Packets of seeds

25 packets cost ৳16 each. 25 × 16 looks hard… But 25 × 4 = 100, so 25 × 16 = 100 × 4 = 400!

Double and halve

Doubling one factor and halving the other keeps the product. 25 × 16 = 50 × 8 = 100 × 4. Keep going until it is easy.

Split it

Split a number into tens and ones. Multiply each part. Add: this is the distributive law.

Decimals

0.3 × 4: 3 × 4 = 12, then divide by 10 → 1.2. × 0.5 is the same as halving. 0.6 × 0.5 = 0.3.

What I learnt

• I can double and halve.
• I can use the distributive law.
• I can multiply decimals by whole numbers mentally.