More work on doubling and halving, including larger numbers and fractions.

When we are multiplying by a number ending in 5 we can double the number and halve the other number before multiplying (by a multiple of ten, making it easy).

To multiply by 21, just multiply by 20 and then add the number on. (To multiply by 20, double and times by 10.)

When multiplying 2-digit numbers by 1-digit in your head usually it is easier to start with the tens.