A man rows downstream at 20 km/hr and rows upstream at 15 km/hr. At what speed he can row in still water?
17.5 km/hr
18.5 km/hr
19.5 km/hr
20 km/hr
Explanation:
Speed of stream =$\dfrac{1}{2}\left(Downstream\:Speed+Upstream\:Speed\right)$
Speed downstream = 20 km/hr
Speed upstream = 15 km/hr
Required speed =$\dfrac{1}{2}\left(20+15\right)km/hr$
=$\dfrac{1}{2}\times 35$
=17.5km/hr