2 Apr 2013

Mental Maths: Multiplication Test

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This is a useful trick to help kids learn their times tables, factorise - or divide a fixed quantity of ducks (for example) between three people...

How can you tell if a number will divide by three without using a calculator?


Do this:
Add up the digits like so:

24? 2 + 4 = 6

54? 5 + 4 = 9

1056? 1 + 5 + 6 = 12


If the number you came up with will divide by three, then the first number will too. You can share twenty four ducks perfectly between three people. Same goes if you have one thousand and fifty six.

You can do this for six too. If the number's odd, you can rule it out straightaway. So, if it's even, first split the number in half. Now add up the numbers as above.


Hurrah! Go make new friends and impress the ones you've already got with your new superpower.

So

1056 ÷ 
2 = 528

528
5 + 2 + 8 = 15 (so 1056 will divide by six!).

Actually when I tried to amaze my son with this, he just laughed in my face. He'd already done it at school. Which probably explains why his mental maths is so bafflingly good.