Saturday, February 04, 2006
You can multiply any two numbers between 6 and 10 on your fingers!
- Represent each finger numerically as 6, 7, 8, 9, and 10. (e.g. your thumb is 6, your forefinger is 7, your main finger is 8, etc)
- Cross the fingers representing the numbers you're multiplying on each hand, and do the same for all the numbers lower than them. (for example, if you're multiplying 7 x 8, cross the first three fingers of your right hand with the first two finger of your left hand)
- All the crossed fingers represent 10's. Add the number of crossed fingers.
- Multiply together the uncrossed fingers on each hand and add it to the sum of the crossed fingers (which represented 10's, remember)
So, if we're multiplying 7 x 8, you have two fingers crossed over three fingers. That is (2 + 3) x 10 = 50. The uncrossed fingers are multiplied and equal 3 x 2 = 6. The final solution is 50 + 6 = 56, which is the solution to 7 x 8. It's easier to do than it is to describe.
I have also proven that this works no matter how many fingers you have, as long as your hands are symmetric, and that your number base system is the same as the number of fingers you have. I'll update with the proof later, and maybe an illustrative picture.