Will a number divide by 11, 7 or 13? 3vp5c

Here are three little tricks which could save you a lot of grief if you're a tough person who doesn't use calculators. The 13 test   2z1o52

Will a number divide by 11? g6n4f

There's a quick way to know if a number will divide exactly by 11 with no remainder - and not many people know it! 4h3l3

What you do: Put (or imagine) alternate + and - signs in front of the digits Total them all up If the total is zero, or if the total divides exactly by 11, then the original number will divide by 11. 5x425f

Let's try one...91825074 Yuk! But will it divide by 11? Put the signs in: +9 -1 +8 -2 +5 -0 +7 -4 Get all the "+" bits and "-" bits together if you like: +9 +8 +5 +7 - 1 -2 -0 -4 This becomes: 29 - 7 = 22 22 divides by 11, so that means 91825074 will divide by 11 too! If you try 17483103You get +1 -7 +4 -8 +3 -1 +0 -3 which makes -11. That still counts as dividing exactly by 11, so 17483103 divides by 11.What use is all this?If you get around to breaking massive numbers into prime factors (this comes up in "The Mean and Vulgar Bits") this trick can save a lot of time!Another time this comes in handy is when you're the all supreme ruler of a massive planet and the entire population wants to know if they can all be divided neatly up into football teams. n12d

The 11 TEST (With a pretty counter!)

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Will a number divide by 7?

On page 111 of "Murderous Maths" you'll find a the long way of testing if a number will divide by 7. However since the book came out, we've found a slightly quicker way! Take off the last digit and multiply it by 2 Subtract it from the digits you have left. If the answer divides by 7 (or is zero), then your original number will also divide by 7! 4q486c

If your number has three or more digits, you could also try this brilliant "7 test" that was sent to us by a Murderous Maths fan in Kent called Jaimie Oldham. Take off the last two digits then multiply the other digits by 2. Add your answer to the two digits you took off. If the answer divides by 7 then your original number will also divide by 7! 3x4q3e

When you do any "7" tests, it's handy to that 77, 84, 91 and 98 all divide by 7. 452w68

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 Will a number divide by 13?

This is similar to seeing if a number will divide by 7. Take off the last digit and multiply it by 9 Subtract it from the digits you have left. If the answer divides by 13 (or is zero), then your original number will also divide by 13! 475w31

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