D is also for… Division

D is for...Sometimes in maths it’s really useful to be able to look at a number and tell quickly what numbers it is possible to divide it by. There are a few tricks you can use to tell whether one number is divisible by another , so I’ll share them with you.

 

How can you tell if a number is divisible by 1?

It’s simple. All numbers are divisible by 1.

How can you tell if a number is divisible by 2?

This is simple too. If it’s an even number, it’s divisible by two; if it’s an odd number, it’s not.

How can you tell if a number is divisible by 3?

Add up all the digits until you get a single digit number, If it is 3, 6 or 9 then the number is divisible by 3. For example: 462         4 + 6 + 2 = 12           1+2 = 3. So 462 is divisible by 3. 729          7 + 2 + 9 = 18          1 + 8 = 9. So 729 is also divisible by 3.

How can you tell if a number is divisible by 4?

Look at the last two digits. Halve them. If you get an even number then it’s divisible by 4. For example   13,564         the last two numbers are 64. Half of 64 is 32 which is an even number so 13,564 is divisible by 4.

How can you tell if a number is divisible by 5?

This one’s a bit easier. If it ends in a 5 or 0 then it’s divisible by 5.

How can you tell if a number is divisible by 6?

If it’s an even number, and it’s divisible by 3 (see how can you tell if a number is divisible by 3) then it’s also divisible by 6.

How can you tell if a number is divisible by 7?

This one is a bit tricky, so bear with me. Take off the last digit and double it. Take it away from the rest of the number. If the answer you get is divisible by 7, then the whole number is divisible by 7. If you’re not sure then take off the next digit and repeat the process. This method definitely needs an example!

833             take off the last digit (3) and double it = 6

83 – 6 = 77

77 is divisible by 7 so 833 is also divisible by 7.

3192          take off the last digit (2) and double it = 4

319 – 4 = 315

Not sure whether 315 is divisible by 7, so take off the last digit (5) and double it  10

31 – 10 = 21 which is divisible by 7 so 3192 is divisible by 7.

How can you tell if a number is divisible by 8?

Look at the last three numbers. Halve them and halve them again. If you get an even number then it’s divisible by 8.

How can you tell if a number is divisible by 9?

Follow the procedure for telling if a number is divisible by 3. If the single digit you get is 9, then the number is divisible by 9. In the example given in how to tell if a number is divisible by 3, 729 is divisible by 9, but 462 is not.

How can you tell if a number is divisible by 10?

If it ends in a 0 then it’s divisible by 10.

How can you tell if a number is divisible by 11?

Alternately subtract then add the digits. If the answer is a multiple of 11 (including 0) then the number is a multiple of 11.  This one needs an example too!

6425936 –> 6-4+2-5+9-3+6 = 11, so 6425936 is divisible by 11.

How can you tell if a number is divisible by 12?

If it is divisible by both 3 and 4 (see above for how to tell) then it’s divisible by 12.

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