Amazing Number Facts No

No 21

How can you tell if a number divides exactly by 2?
Answer: If it ends in an even number.

How can you tell if a number divides exactly by 5?
Answer: If it ends in a 0 or a 5.

These facts are very familiar however...

How can you tell if a number divides by 11?
Answer: Alternately add and subtract the digits from right to left.
If the answer divides by 11 then so does the original number

Let's check 1999: 9-9+9-1=8 so it doesn't divide by 11.
Let's check 2013: 3-1+0-2=0 so it does divide by 11 ( since 0 divides exactly by 11).
Let's check: 723456789: 9-8+7-6+5-4+3-2+7=11 so it does divide by 11.

Now try some of your own... If you have answers, discoveries, new questions etc to do with this Number Fact then...
E-mail us at madrascollege.maths@fife.gov.uk
...and we will publish them here.