If I had a dollar for every time a student in a math class has asked me "how do I use this in real life?" I'd have a paycheck. But, like it says in the song, "if I had a million dollars, I'd be rich."
I'm asked this most often by Algebra 2 and Geometry students, so here are three things I can show them.
(1) The Rule of 72. Which states, in probably over-simplified form:
"If you take the number 72, and divide it by your interest rate, the result is the number of years it takes for the principal to double."
That's a pretty good simulation of compound interest there, especially for rates around...