三下五除二

Etymology
A mnemonic device to aid with adding 3 to 2, 3 and 4 in base 10 (also 7, 8 and 9 in base 16) on an abacus.

For example, to calculate $$3+3$$:
 * 1) Start with three beads on the bottom (each lower bead is equal to 1).
 * 2) Move one of the top beads down (each top bead is equal to 5).
 * 3) Take away two of the beads from the bottom.
 * 4) This leaves you with one top bead (5) and one bottom bead (1), for a total of 6.

Idiom

 * 1)  (doing things) in an instant; in the blink of an eye; quickly and easily