One of my colleagues, I will call X, was helping a friend of his shop for a car. The price was reached and financing discussed. A printout of payments was given to the friend, and X asked for a copy. He then asked to see the manager.
He informed the manager that he was taking this piece of paper to his lawyer and planned to sue. The interest rate that was advertised and told to the buyer was 1.9%. The interest rate that made these payments was 4.9%. The manager said he would recalculate. X informed the manager that the other dealership under the same management was advertising 0.9% and that’s what he expected his friend to get. The contract was written that way. X saved his friend quite a bit of money, because he knew math and brought his calculator. I wonder how many people actually paid 4.9% interest without realizing it. Probably quite a few.
On the other hand, I met a former student who told me she used some of the math she learned from me in a video game. That’s not really practical math.