Explanation:
72÷9= 8, so the boat was only doing 8mph going upstream (against the current) the return trip downstream only took 4 hours. 72÷18=4 meaning the boat was traveling at 18mph downstream (no current) there is a 10mph difference between upstream and downstream therefore the current had to be running at 10 mph. so the speed of the tug boat is 8mph without a current and the current is running at 10mph.