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.