I drove to the beach at a rate of 40 miles per hour. If I had driven at a rate of 50 miles per hour instead, then I would have arrived 30 minutes later. How many miles did I drive?
Let the distance to the beach==D
Let the time taken to drive to the beach ==T
D /40 == T
D /50 == T - 1/2, solve for D, T
D==100 miles - distance to the beach
T ==5/2 ==2.50 hours - time it takes to drive to the beach
Note: if you drove at 50 mph, you would get to the beach 30 minutes EARLIER and not LATER !!!