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Comment by joshmn | Hacker News Reader

My favorite is still the multiples of 9 by putting down one finger. It amazes me.

Of course it only works with a full set of fingers, but still.

A fun property I learned that helps me remember the 9s: The digits of every multiple of 9, when totaled, eventually add up to nine.

For 9 times x, for 2-10 at least, this can be reverse engineered: The 10s digit of the product will be x-1, and then the 1s digit will be whatever number remains that's needed for the digits to add up to 9.

So 9 times 7: 7-1 = 6, 6+3 = 9, therefore 9*7 = 63.

There may be a similar trick for bigger numbers, but I rarely need to do them quickly so I've never found one.

Multiply by x by ten, substract x?

I was wondering why no one was suggesting this, isn't this the most obvious one?

I always found 9 to be easy because whatever you multiply with (2-10) you just had to remove 1 from that number, and maybe memorise the second digit.

e.g.

9*3 = 3-1 = 2 (+ the 7 that I just memorised)

But actually the finger trick is amazing, and I just used it to double check the numbers as it was a loooong time I did it.

The "remove 1" trick is sort of how I learned it, except instead of memorizing "2 goes with 7" I learned that the 2 digits should add back up to 9:

  9*3 = 27 (3-1 = 2,7 = 9-2)
  9*4 = 36 (4-1 = 3,6 = 9-3)
  ...
  9*9 = 81 (9-1 = 8,1 = 9-8)

Fun stuff :)

9s are easy if you know how:

http://www.trachtenbergsystem.org/multiplication-9/

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