## Description and game instructions

If you do not already know how to use an abacus you are recommended to play the game Abacus - basics. There you may also find a description of how the abacus is built up and the beads values. If you want to practice plus before minus there's a suitable game called abacus - addition.

#### Difficulties in performing subtraction

When you calculate subtraction on an abacus there are two difficulties you may encounter as described below. There are corresponding levels in this abacus game, to be able to practice them all.
##### Switch five
Carrying five means that there are not enough beads in the lower deck of the rod and you need to switch into a bead with the value of five from the upper deck. It is the same type of switching as when performing addition, but the subtractions you need to know in these calculations are:
 -4 = -5 + 1 -3 = -5 + 2 -2 = -5 + 3 -1 = -5 + 4
##### Switch ten
Carrying ten means that there are not enough beads on that rod. Therefore, a bead from the rod to the left need to be switched. Try to make the switch directly - without switching between the two beads above the beam (which each have the value five). You have to master the following additions to perform a carry with the value of ten:
 -9 = -10 + 1 -4 = -10 + 6 -8 = -10 + 2 -3 = -10 + 7 -7 = -10 + 3 -2 = -10 + 8 -6 = -10 + 4 -1 = -10 + 9 -5 = -10 + 5
##### Switch both 5 and 10
This means you have to perform both the switched mentioned above. At first it might be difficult to manage the switches and it's really easy to get lost in an exchange. One tip is to remember the initial value the abacus had so that you can start over just from there, without having to restart the entire round.

### Calculation of knowledge points

Each completed level gives you one knowledge point. The maximum number of points (12 knowledge points) is achieved when you complete twelve levels. Even when you have completed a level, you can continue practicing at that level, but it gives you no more knowledge points.