A man can row at 5 km/hr in still water, if the river is running at 1 km/hr it takes him 75 minutes to row to a place and back. How far is the place?

A. 2.5km
B. 3km
C. 4 km
D. 5 km
Explanation:
Speed downstream = (5+1)km/hr = 6 km/hr
Speed upstream = (5-1)km/hr = 4 km/hr
Let the required distance be x km.
Then x/6 + x/4 = 75/60 = 5/4 => (2x + 3x) =15 => x =3.
Required distance = 3 km