A boat covers 48 km upstream and 72 km downstream in 12 hours, while it covers 72 km upstream and 48 km downstream in 13 hours The speed of stream is :

Answer: Option A

Let downstream speed be D and Upstream speed be U. Then
$$\begin{array}{rl}& \frac{48}{U}+\frac{72}{D}=12.......\left(1\right)\\ & \text{and}\\ & \frac{72}{U}+\frac{48}{D}=13.......\left(2\right)\end{array}$$
On solving the equation (1) and (2), we get
D = 12 kmph and
U = 8 kmph
Thus, speed of the current
$$=\frac{D-U}{2}=\frac{4}{2}=2\text{kmph}$$