average speed in MPH

Question

A boat traveled downstream a distance of 72 mi and then came right back. If the speed of the current was 5 mph and the total trip took 6 hours, find the average speed of the boat relative to the water

Answer 1

When traveling downstream, the speed of the boad relative to the water is 72-5 = 67, on the way back it is 72+5=77. The average speed for the whole trip is (67+77)/2=72 which is the same as the original speed of the boat.