Answer:
Option A
Explanation:
Speed of boat in still water = 10 km /h
Let rate of flow of river $=y$ km/h
Total time = $\frac{Total\quad Distance} {Average\quad Speed}$
= $\frac{Total\quad Distance\times Speed\quad in\quad still\quad water} {Upstream\quad Downstream}$
$20=$ $\left(\frac{2\times 91\times 10} {(10+y)(10-y)}\right)$
$y$ = 3 km/h
