In a stream with a current of 5 kilometers per hour a boat made a trip downstream from point A to a point B and then back upstream from point B to point A. Measured in still water, the boat’s average speed for the trip was 15 kilometers per hour. Measured in the stream, what was the boat’s average speed for the trip in kilometers per hour?
Please explain your answer.
Optional: time you spend on this question
Let distance from A to B = S
downstream speed = 15+5=20 .... time used = S/20
upstream speed = 15-5=10 .... time used = S/10
average speed = total distance/total time spend
=2S / (3S/20)
= 13 1/3
edit for typo