The following list shows how you can use DOS to do basic arithmetic, for example sum, subtraction etc.
You can use these with other variables, for example if I hate a variables called %number% which was 11, I can use the following command to make it 10
SET /A Result = 12 + 4
SET /A Result = 23 - 7
SET /A Result = 8 * 2
SET /A Result = 33 / 2