You can use the SetValue function to save to a new or existing variable.
Let's assume you have a variable called "Q1" and you want to apply the arithmetic you mentioned.
Go to the Pass-In Fields and define a variable as a Decimal Number. Call it "Q1NEW".
Pop this script into an appropriate question footer (one that is not being skipped).
This will take Q1 and minus 10, then divide that answer by 10, and then save it to a new variable called Q1NEW.
Replace Q1 and Q1NEW with your variable names.
You can check within the admin module the values of Q1 and Q1NEW as you are testing the survey.
And it is good practice to keep the original variable and the calculated variable separately.