A man rows at a speed of 6 km/hr in still water. If the time taken to row a certain distance upstream is 4 times the time taken to row the same distance downstream, what is the speed of the river?

A. 1.8 km/hr
B. 3 km/hr
C. 3.6 km/hr
D. 4 km/hr
Let x be the speed of the river.
Ds = (6 + x) km/hr; Us = (6 – x) km/hr
If t hours is the time to row downstream then 4t hours is the time to row upstream.
(6 + x)*t = (6 – x)*4t
6 + x = 24 – 4x
x = 3.6 km/hr