Halving and doubling
You learn a trick to make multiplying easier: halve one number and double the other.
How to work it out
- Halve the even number and double the other number.
- The answer stays the same, but it is much easier to calculate!
14 × 15 = 6022 × 30 = 60
A worked example
24 × 5 = ?
- Halve 24: 12.
- Double 5: 10.
- Now: 12 × 10 = 120.
Another example
16 × 25 = ?
- Halve 16: 8.
- Double 25: 50.
- Now: 8 × 50 = 400.
Try it yourself
18 × 25 =
4 × 35 =
Watch out
You halve both numbers, and then the answer is no longer right: 4 × 35 becomes 2 × 17.5.
One number goes down and the other goes up by the same amount. Halve one, double the other.
You halve an odd number, like 35, and end up with a decimal.
Always halve the even number. If there is no even number, choose another strategy.
Tips
- This trick only works if one of the numbers is even.
Why it works
If one number is even, you can halve it and double the other. The answer is the same. 24 × 5 is the same as 12 × 10. That is easier to work out.