Murderous Maths
The 7-11-13 trick! (And other miracle sums)

This trick makes you look like your brain is a mega fast calculator!

Ask a friend to write down ANY three digit number such as 231 or 884. Ask them to multiply the number by

Easy enough for you? ...but even if your friend has used a calculator, you will have written down the answer ages ago! It's fiendish!

THE SECRET: all you do is write out the starting number twice! So 231 will become 231231 and 884 will become 884884. You don't believe it? Well try it on this calculator and see for yourself! You work this calculator by clicking the mouse on the buttons. Go on, put in ANY three digit number then x7 x11 x 13 and see what you get!

The 3-7-13-37 trick:

This trick is very similar to the 7-11-13 trick.

  • Get a friend to pick any 2 digit number e.g. 49
  • x 3
  • x 7
  • x 13
  • x 37
  • The final answer will be the original number written out three times! So 49 x3 x7 x13 x37 = 494949

The 3367 trick:

This trick is similar to the 3-7-13-37 trick. It's harder to do, but it looks far more miraculous!

  • Get a friend to pick any 2 digit number e.g. 74
  • x 3367
  • To work out the final answer you have to imagine the original nunmber written out three times e.g. 747474 then divide it by three. 249158
This one takes practise, but unlike the others, it's very hard to see how it's done!

The 9091 trick

One of our MM fans who calls himself TOMATH told us about an even bigger version of this trick:

  • Take any 5 digit number e.g. 76543
  • times it by 11
  • then multiply the answer by 9091
  • The final answer will be the original number twice! So 76543 x11 x9091 = 7654376543

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