Today with my class of 10 year olds, we continued discovering why maths is great. Or rather, they continued humouring me while I got over-enthusiastic about numbers. We found out that:
e.g. 45 4+5=9, so therefore 45 is divisible by 3.
1542 1+5+4+2=12 1+2=3, so therefore 1542 is divisible by 3.
This is just the beginning…
e.g. 203 : 3 doubled is 6, 20-6=14, so 203 is divisible by 7.
7735 : 773-(5×2)=763, 76-(3×2)= 70 which is divisible by 7, so so is 7735.
e.g. 365167484
3-6+5-1+6-7+4-8+4=0 so it is a multiple of 11.
914682538
9-1+4-6+8-2+5-3+8=22 so it is a multiple of 11.
Oh the fun! If you reached the end of this post, thanks for humouring me too.
NB: adding together the digits of a number finds that number’s ‘digital root’ (e.g. 12 = 1+2 = 3.) If you get a multi-digit number when you do this, you keep adding the digits together until you reach a single digit answer. Find the digital roots of the answers in all of the times tables and you’ll find loads of patterns!
Tags: divisibility, key stage 2, maths, multiple, primary school
Wow! I knew none of those tricks! (But I know plenty of others – have you read Alex Bellos’ “Alex’s Adventures in Numberland”? It’s fantastic!)
I did know most of these, but I do like them. I tried to prove a couple of them once and ended up with a headache.
Maths is fun!