Most of the math I do at work is related to compound interest. Of all the math I believe the general public should understand, the concept of how paying interest to others is a total screw would get my top vote.

I have a co-worker who took out a car loan last week at, wait for it, FIFTY THREE PERCENT INTEREST! No concept of what that was costing her. She could only see, “I can afford the monthly payment.”

(1 + r)^n and its friend 1/(1 + r)^n have been the two most important concepts in work and personal life that I’ve ever learned and applied.