The Math of Sales

This isn't as nerdy as it may seem:


I have to admit, the center panel never occurred to me.  Not even in the abstract, let alone as a simple formula.  The other two, though, are near and dear to my heart.  "Up to X or more" always struck me as an offense against English, never mind math.  The third panel reminds me of the first four or five years of married life.  Lori would come home with some clothes or shoes or whatever, and announce with delight "This was originally $75, but I paid twenty!" to which I would reply "Great!  So, how much did you save?"  Eventually, we both came to agree that the correct answer is:  NOTHING!  Lesser marriages might have crumbled under this and a dozen other little exchanges of ours, but for us it was always basically a comedy routine for two.