The term 'USED AN ABACUS' can be solved by the five-letter word 'ADDED'. An abacus is an ancient counting tool used for basic arithmetic calculations. It consists of a frame with beads that are moved along rods to represent numbers. The abacus has been used for centuries and is still used in some parts of the world today. Adding numbers on an abacus involves sliding the beads up or down the rods to represent the numbers being ADDED.