A boat can travel at 10 mph in still water. It travels 42 miles downstream with the current in the same time it travels 18 miles upstream. What is the speed of the current?

Solution:

The speed of current: x mph

So the speed downstream is (x+10) mph

the speed upstream is (10-x) mph

So the equation is

(10+x)/(10-x) = 42/18

x = 4

So the speed of current: 4 mph