A boat covers a certain distance downstream in 1 hour, while it comes back in 11/2 hours. If the speed of the stream be 3 kmph, what is the speed of the boat in still water?

Answer: Option D

Let the speed of the boat in still water be x kmph
Then
Speed downstream = (x + 3) kmph
Speed upstream = (x - 3) kmph
$$\begin{array}{rl}& \therefore \left(x+3\right)\times 1=\left(x-3\right)\times \frac{3}{2}\\ & \Rightarrow 2x+6=3x-9\\ & \Rightarrow x=15\text{kmph}\end{array}$$