A motorboat goes 36 miles upstream on a river whose current is running at 3 miles per hour. The trip up and back takes 5 hours. What is the speed of the boat (assuming that it maintains a constant speed relative to the water)?

Solution:

The speed of the boat is x

So

36/(x+3)+36/(x-3)=5

So

x = 15

So the speed of the boat is 15 mph