Money problems tell one little story, and it always has the same four characters. The principal $P$ is the money you start with. The rate $r$ is the percent the bank pays each year — say it per year every time, because a rate is a percent wearing a clock. The time $t$ is how long the money sits, in years. And the interest $I$ is the thank-you the bank adds for holding your money.

They meet in the friendliest formula in this course: $I = P\,r\,t$ — read aloud: I equals P r t. Three things multiplied, nothing else. You will spend this chapter getting to know it from every side.