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There are a lot of math shortcuts that aren't often taught to our students anymore. If you should wish to determine factors of a number, there are some short cuts:

1. All even numbers are divisible by 2. If the last digit is divisible by 2 then the number is even.

2. To determine if a number is divisible by 3, add up the digits. Keep adding until you get a single digit. If this digit is divisible by 3, then the entire number is. For example:

Consider the number 3456. Adding the digits together, 3+4+5+6 = 18. Repeat, 1+8 = 9. 9 is divisible by 3, therefore 3456 is divisible by 3. A bonus - the final number is 9, therefore the original number is divisible by 9.

3. If the number ends in 5 or 0, it is divisible by 5. If it ends in 0, it is divisible by 10.

Another very useful trick is multiplying by 11. There is a simple procedure:

Consider 11 x 26.

Take 26 and separate the numbers. Add 2+6 = 8. Put the 8 in between, giving you 286. This is your answer.

If the number being multiplied has digits that add up to more then 9, you simply carry over to the first digit. For example:

Consider 11 x 28

2 + 8 = 10

add 1 to the first digit, giving 3, and put the 0 in the middle. You end up with 308 as your final answer.

Practise with these tricks will help you feel more confident when working with numbers.

Learn more about this author, Naomi E.
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