Subtracting Variables

I have a feature in Storyline 3 that is a tote board for totaling correct and incorrect answers.  If correct, it goes to a Correct layer where 1 is added by a variable.  Similar if incorrect.  Problem is when they hit PREV.  There seems to be no way to subtract the score they just got without messing up the totals.  I tried adding a trigger to the Correct and Incorrect layers to subtract 1 if they hit PREV, but the triggers jump to the base layer, so hitting PREV subtracts 1 from both totals.  Presumably no one needs to go back if correct but I like to anticipate that they might, so I want this to work properly.  My only solution so far is to disable the PREV button.  Any ideas? 

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