Ishita is rowing a boat. She takes 24 hours to row 72 km against the stream and she takes 9 hours to return to the starting point in the direction of the stream. The speed of the boat in still water is :

$\frac{11}{2} km /hr $

$\text{Distance}=72\text{ km}.$

$\text{Upstream time}=24\text{ h},\;\text{Downstream time}=9\text{ h}.$

$\text{Upstream speed}=\frac{72}{24}=3\text{ km/h}.$

$\text{Downstream speed}=\frac{72}{9}=8\text{ km/h}.$

$\text{Speed in still water}=\frac{3+8}{2}=\frac{11}{2}.$

$\text{Speed of the boat in still water}=5.5\text{ km/h}.$