SOLUTION: The speed of a boat in still water is 9 km/h. The boat travels 72 km upstream and 72 km downstream in a total time of 18 hr. What is the speed of the stream?

Let V be the speed of the stream (of the current), in km/h

Then   the speed of the boat traveling downstream is (9+v) km/h, 
while  the speed of the boat traveling upstream   is (9-v) km/h.

The time for traveling upstream is  hours,
while The time for traveling downstream is  hours.

Therefore, the time equation is 

 = 18   (total time).


Divide both sides by 18 and multiply both sides by (9-v)*(9+v). You will get

4(9+v) + 4*(9-v) = ,   or

 = 81 - 2*36 = 9.


Therefore, v = 3 km/h is the rate of the current.


Answer.  The rate of the current is 3 km/h.