The following list shows how you can use DOS to do basic arithmetic, for example sum, subtraction etc.
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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
Addition
SET /A Result = 12 + 4
Subtraction
SET /A Result = 23 - 7
Multiplication
SET /A Result = 8 * 2
Division
SET /A Result = 33 / 2
Reference: www.robvanderwoude.com/battech_math.php