How to use and Abacus
The most common Abacus is the 2/5 Abacus. This means that there are two beads on above the bar and 5 beads below the bar.
This abacus is in base 10 this means that each rod/column is each of the separate 10s position. For the example below this can represent every number from 0 to 99999.
Starting point
When retesting it to 0 all of the beads will be not touching the mid bar. This means that the top beads are up and the bottom beads are down.
The top beads count as 5x that columns value. The bottom beads count as 1x that columns value.
Adding on the Abacus
When adding a number to the Abacus you start with the highest digit first with the bottom beads first. When you get to a mutable of 5 you reset the lower beads of the column and set a single top bead. When you have both top beads set from a single column then you unset them and go to the column to the left and set a bottom bead. This is just like a regular carryover in addition.
Subtracting on the Abacus
When subtracting a number on the Abacus you start with the highest digit first and remove the beads of the corresponding row. If you cant do that with the beads in the bottom row. You move one of the top rows down and then add 5 beads to the bottom row. If you don’t have any of the 5’s set then you subtract from the row to the left and set one of the 5’s and 5 of the 1’s.
Have fun with the interactive Abacus and see if you can add and subtract numbers on it.