A fisherman traveled in a boat from point N upstream. After having traveled 6 km, he stopped rowing and 2 hour 40 min after first leaving N, he was brought back to N by the current. Knowing that the speed of the boat in still water is 9 km/hour, find the speed of the river’s current.

Answer 1

speed of current ---- x km/h

so speed against the current = 9-x km/h
speed drifting ----- x km/h

time going against the current = 6/(9-x) hrs
time drifting back down = 6/x hrs

total time:
6/(9-x) + 6/x= 2 2/3 = 8/3

multiply each term by 3x(9-x) and solve for x