A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:

Answer: Option B

Let the speed of the stream be x km/hr
Then,
Speed downstream = (15 + x) km/hr
Speed upstream = (15 - x) km/hr
$$\begin{array}{rl}& \therefore \frac{30}{15+x}+\frac{30}{15-x}=4\frac{1}{2}\\ & \Rightarrow \frac{900}{225-x^2}=\frac{9}{2}\\ & \Rightarrow 9x^2=225\\ & \Rightarrow x^2=25\\ & \Rightarrow x=5km/hr\end{array}$$