There is another way of doing math on your fingers, which gives you a much greater range of numbers, and which makes multiplication particularly easy. It’s a bit more work to get used to than the finger abacus, but it has a lot less limitations. Someone in the comments of the finger-abacus post mentioned that they do something similar.
The methods for binary fingermath that I’ll describe are my own creation; so if you think they’re ridiculous, the blame is entirely mine. I know other people have come up with similar things, but this is my own personal variant.
The idea is to use your fingers in binary: the thumb is 1, the pointer is 2, middle finger 4, ring finger 8, and pinky 16. With this, you can get to 31 on one hand, or 1023 with both. So, here’s a diagram of a few different numbers on the right hand:
We’ll start with the simplest version of addition – this can be done on one hand, but it’s easiest to use two hands, with one as a placekeeper. The basic algorithm is:
1. Put one number on each hand in binary.
2. Go to the lowest raised finger on the left hand:
1. Lower that finger on the left hand.
2. Add one starting at the same finger on the right hand:
1. If the finger is lowered, just raise it.
2. If the finger is *raised*, then lower it, and add one starting
at the next finger.
3. Repeat step 2 until the right hand is zero.
So, here’s an example of finger-binary addition:
For larger numbers, I like to use a stack of pennies. Lay out ten pennies in front of you. Put one number on your hands – do the other number using the pennies – push pennies away from you for a one, towards you for a zero. The algorithm remains the same – except that now you can do additions up to 1000.
You can also do binary multiplication on your fingers, but that’s a topic for tomorrow.