Especially math tricks. Like these divisibility tricks:

**2**

We all know that even numbers are divisible by 2** **right? Easy.

**3**

Similarly, you can tell if a number is divisible by three if the sum of its digits is divisible by three. For example:

156 is divisible by 3 because 1 5 + 6 = 12, which is divisible by 3.

Cool, eh?

**4**

We can tell if a number is divisible by 4 if the last two digits are divisible by 4:

135,624 is divisible by 4 because the last two digits (24) are divisible by 4.

**5**

We know that a number that ends with 0 or 5 is divisible by 5 (easy).

**6**

A number that is divisible by 2 and 3 is also divisible by 6. Example:

9,876 is divisible by 2 because it’s en even number.

9,876 is divisible by 3 because 9+8+7+6 = 30 which is divisible by 3.

So, 9,876 is also divisible by 6.

**7**

7 is a little trickier:

If you take the last digit in the number, then double and subtract it from the rest of the digits, if the result is divisible by 7, so is the number.

Example: 364 (double the 4 to get 8. Subtract 8 from 36 = 28. 28 is divisible by 7, so 364 is as well.

Another way to figure it out (but somewhat more work) is:

Take the number and multiply each digit starting from the right (ones) by 1, 3, 2, 6, 4, 5… then add the products. If the sum is divisible by 7, so is the number. Example:

2072: 2(1) + 7(3) + 0(2) + 2(6) = 2 + 21 + 0 + 12 = 35 which is divisible by 7, so 2072 is too.

**8**

A number that ends with 3 zeroes is divisible by 8. Also, if the last 3 digits are divisible by 8, so is the whole number.

1,234,567,000 is divisible by 8. And so is 999,880.

**9**

Same as the 3 one, if the sum of the number’s digits is divisible by 9, then so is the number. For example:

1,234,323 = 1+2+3+4+3+2+3 = 18 which is divisible by 9, and so is 1,234,323

**10**

Easy. Ends in zero = divisible by ten.

**11**

Another one of my favourites has to do with multiplying by 11. A quick trick to multiply a 2-digit number by 11 is to add both digits to form the middle digit of the answer.

Example:

11 x 14 = 154 because 1+4 of 5 and makes up the middle number.

11 x 27 = 297 because 2+7 = 9 and makes up the middle number.

It gets a bit trickier when the two digits are larger than 9 bu it’s still doable:

11 x 85 = 935 because 8 + 5 = 13 but you then have to add the extra 1 to the 8 like:

8

13

93 then tack on the 5 for 935.

And so on.

Cool eh?

on July 24, 2010 at 10:21 am |1domesticgoddessOk, I just have to say this, my eyes just crossed! I have never been one for math (sigh) I wish, but it’s just not the way my poor little peabrain works!