I use the Present Value (PV = FV * 1/(1+r)^number of periods), and Future Value (FV = PV*(1+r)^number of periods formulas all the time, I cannot forget them. They are very handy when trying to explain to you kids how much my first car's sound system was in TODAY'S DOLLARS, and stuff like that. Or, how much this old house will be worth when your mother and I die (even though I will sell this thing basically as soon as you go off to college and we will move back to Lawrence.