11 plus maths: Tried the quick double and half trick for long multiplication?

Here’s a tip fresh from a lesson with a student this morning. Can you see what’s going on in the example in the box?

double and half

It doesn’t replace long multiplication. However, there are times that numbers look a bit yucky, but with this simple trick, they become numbers our brains love. Above, we changed 225 and 28 into incredibly friendly times tables – either 9 x 7 or 7 x 9.

If your child is pressed for time and is confident with doubling and halving, this trick can save seconds and reduce the risk of long multiplication errors. Three things to note:

  1. When you half one number and double the other, the answer is always the same. You can see this with a simple example:  2 x 4 is the same as 1 x 8. We halved the first and doubled the second.
  2. You can’t half an odd number. (Well, you can, but this would make it way too complicated, which means this trick would not be wroth bothering with for that question.)  So if you have two odd numbers to multiply, go for long multiplication. If you have two even numbers, or an even and an odd, then it might work. In our box above, we stopped when we reached the odd number 7.
  3. There is a lot of value – in terms of gaining speed and developing the mental maths muscle – in practising doubling and halving lots of different numbers. You can easily ask your child a question or two in daily conversation.

I hope you and your child find value in the above tip. For lots more happy ways to help your child be ready for the big day, keep following and stay 11plushappy!

You might also wish to make this brand-new 11plus book series a part of your learning strategy.