I heard that this method has been known since the Vedic age. It might has been known much longer (we can’t decipher Indus script yet).
You can count between 0 and 15 (0 to F in hex) on one hand or between 0 and 255 (00 to FF in hex) on two hands.
A horizontal move to the neighboring finger on the left adds or subtracts 4 (depending on which direction you move); while the same move on the right hand adds/subtracts 40 in hex (assuming that your count the least significant digit on the left and the most significant digit on the right; you can do the other way around as well).
For example, lets calculate 3A + 4B.
Result: 3A + 4B = 85. Now, let’s practice subtraction. For example, let’s calculate F1 – 76.
To subtract 70, just move horizontally two fingers left (which subtracts 80) than go one digit back.
As 6 = 4 + 2, to subtract 6, make one horizontal and two vertical jumps.
Result: F1 — 76 = 7B.
If you want to count numbers larger than 255, grab your younger sister, quickly teach her counting in hex on fingers, and you can easily calculate between 0 and 65535 (0000 — FFFF).
This is how we did ABCD + 1234:
You can add the least significant digit first, or the most significant first, it really doesn’t matter. Just do it accurately.
ABCD + 1234 = BE01