A boat can travel at a speed of 28 km/h in still water. If the speed of the stream is 4 km/h, what is the average speed (in km/h) of the boat if it travels 192 km downstream, stops there for one hour and then returns to the starting point?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e 25.6.
Boat speed in still water = 28 km/h and Stream speed = 4 km/h
Downstream speed = 32 km/h and Upstream speed = 24 km/h
Time downstream = 192 ÷ 32 = 6 hours
Time upstream = 192 ÷ 24 = 8 hours
Stop time = 1 hour
Total time = 6 + 1 + 8 = 15 hours
Total distance = 192 + 192 = 384 km
Average speed = 384 ÷ 15 = 25.6 km/h