I drove in the beach at a rate of 40 miles per hour. If I had driven at a rate of 20 miles per hour instead, then I would have arrived 20 minutes later. How many miles did I drive?
Let the time it took you to drive to the beach ==T
40T ==20[T + 20/60], solve for T
T ==1/3 of an hour - or 20 minutes
1/3 x 40 ==13 + 1/3 miles - that you drove.