Two people start walking at the same time in the same direction. One person walks at 8 mph and the other person walks at 2 mph. In how many hours will they be 4.5 miles apart?

Time = distance/speed

The distance between the walkers increases at the rate of (8 mph) - (2 mph) = 6 mph. That distance will be 4.5 mi after ...

(4.5 mi) / (6 mi/h) = 4.5/6 h = 3/4 h

In 3/4 hour they will be 4.5 miles apart.