math tricks

Multiply by 11

Take the original number and imagine a space between the two digits (in this example we will use 52:

5_2

Now add the two numbers together and put them in the middle

5_(5+2)_2

That is it - you have the answer: 572.

If the numbers in the middle add up to a 2 digit number, just insert the second number and add 1 to the first:

9_(9+9)_9

(9+1)_8_9

10_8_9

1089 - It works every time.


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Quick Square

If you need to square a 2 digit number ending in 5, you can do so very easily with this trick. Mulitply the first digit by itself + 1, and put 25 on the end. That is all!

252 = (2x(2+1)) & 25

2 x 3 = 6

625


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Multiply by 5


Take any number, then divide it by 2 (in other words, halve the number). If the result is whole, add a 0 at the end. If it is not, ignore the remainder and add a 5 at the end. It works everytime:

2682 x 5 = (2682 / 2)

2682 / 2 = 1341 (whole number so add 0)

13410

Let’s try another:

5887 x 5

2943.5 (fractional number (ignore remainder, add 5)

29435



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Multiply by 9

This one is simple - to multiple any number between 1 and 9 by 9 hold both hands in front of your face - drop the finger that corresponds to the number you are multiplying (for example 9×3 - drop your third finger) - count the fingers before the dropped finger (in the case of 9×3 it is 2) then count the numbers after (in this case 7) - the answer is 27.


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