“You do the math.” I can’t say when I first heard this term but it seems to be fairly new. That said, it is being used incorrectly a lot in my opinion.
I would imagine that originally it mean to compare numbers. An example might be that if you buy one or two of an item compared to a dozen. Ears of corn or doughnuts, maybe? If you do the math, it is better to buy the larger number if you do the calculations.
Now people throw the phrase in even if the situation is not one of numbers. I was reminded of this when I heard an advertisement as I was driving around today. The pitch was for some kind of substitute cigarette. They were trying to convince potential buyers that their product was healthier than smoking.
I almost drove off the road when the voice-over said something about the fact that the company could not claim the product was health, but we as listeners could “do the math.”
Hello—there were not figures involved!