Word problems often ask you to solve problems that involve speed or simple interest rates. Here are two rate formulas you should commit to memory:

**Simple interest:***I=Prt*, where*I*represents the amount of interest,*P*is the principal (the initial amount invested),*r*is the interest rate, and*t*is the length of time the money is invested.**Distance:***d=rt*, where*d*represents the distance traveled,*r*is the rate (speed) of travel, and*t*is the amount of time traveled.

In a rate, you can generally think of the word per as a division sign. For instance, suppose someone drives 141 miles in 3 hours, and you have to find the average speed. You want the rate of speed in miles per hour, so you take miles (distance) divided by hours (time): 141 miles ÷ 3 hours = 47 miles/hour. Using algebra, you can rearrange the distance formula to say the same thing:* d* ÷ *t* =* r*.