On the opening screen you will find that your mouse pointer turns into a hand over the three types of problems you can choose. Choose a type of subtraction sum that you would like to try ranging from single to three figure problems.
Once you have clicked on your choice, a random question will appear which you have to solve by clicking on the numbers in green in the white boxes at the bottom of the blackboard. Sums in the second and third level have a green box around the column of numbers that you should solve first. They also have a Borrow One button below the sum's bottom line, which is explained below. You will be told visually and audibly whether you have got the answer correct or not. When you are able to try the next sum, the Next Sum button will become active at the bottom of the board. At anytime you can try a new level by clicking on the New Level button at the bottom.
Borrow One button
If the number at the top of a column is smaller than the one at the bottom, then it is not possible to take away one from the other. Therefore, in order to make the top number bigger you "Borrow One" from the top of the column to the left. This has the effect of adding 10 to the number at the top of the column you are working in. At the same time, you reduce the number at the top of the column to the left by one. When you click on the Borrow One button, you will see that the above is done automatically. Another method is instead of taking one from the top number of the column to the left, you can add or Pay Back one to the number at the bottom of that column to the left.