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 $ $\frac{5}{2} km /hr $ $\frac{11}{4} km /hr $ $\frac{5}{4} km /hr $

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