Think Like a Maths Genius

I’m still enjoying going through Arthur Benjamin and Michael Shermer’s Think Like a Maths Genius. It’s full of great tricks.

For example, how to multiply anything by 11.

We all know how to do this with one digit – you just repeat the number.

7 x 11 is 77

3 x 11 is 33 etc

But Benjamin and Shermer show a trick for two digit numbers.

e.g. 34 x 11

The answer – I can tell you instantly – is 374

All you do is add the two digits together (3+4 = 7) and put that number between the two original digits:

3 & (3+4) & 4

= 374

Another example:

81 x 11 = 891

How?

8 & (8+1) & 1 = 891

As long as the two digits that go in the middle add up to less than 10 it works every time.

But what do you do when the two digits add up to more than 10? Can you work it out?

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5 Comments

Filed under Little things

5 responses to “Think Like a Maths Genius

  1. cfg

    Wow, good to know!
    I need a few tricks up my sleeve when I need to convince people I know something about maths…
    If I read a good non-fiction book I’ll let you know… (does listening to the audio book count?)

  2. cfg

    p.s. Enjoy The Far Pavilions! Lots of true-story stuff in it, but don’t think it counts… 🙂

  3. Pingback: Think Like a Maths Genius part 2 | Nonfiction Book Club

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