2.3 Mental multiplication and division strategies

Doubling and halving are easy for most people, so make use of these tactics:

• doubling and doubling again is the same as multiplying by 4

• doubling and doubling and doubling again is the same as multiplying by 8.

Use the inverse of doubling (i.e. the opposite of doubling) which is halving:

• halving is the same as dividing by 2

• halving and halving again is the same as dividing by 4

• halving and halving and halving again is the same as dividing by 8.

Once we begin talking about halving being the same as dividing by 2 this connects to fractions, decimals and percentages. There will be more of this in Week 2 but for now:

• half of 100% is 50% → to find 50% of an amount just half it

• half of 50% is 25% → to find 25% of an amount half and half it again.

Listed below are some useful mental multiplication strategies:

Multiply by 5 by multiplying by 10 then halving

Multiply by 20 by multiplying by 10 then doubling (or vice versa)

Multiply by 50 by multiplying by 100 and halving (or vice versa)

Multiply by 11 or 9 by multiplying by 10 and adding or subtracting

Multiply by 21 or 19 by multiplying by 20 and adding or subtracting