A boat covers 24 km upstream and 36 km downstream in 6 hours, while it covers 36 km upstream and 24 km downstream in 61/2 hours. The speed of the current is?

Answer: Option B

let speed of boat in still water = x km/h
Speed of stream current = y km/h
According to question,
$$\begin{array}{rl}& \frac{24}{x-y}+\frac{36}{x+y}=6h......(i)\\ & \frac{36}{x-y}+\frac{24}{x+y}=\frac{13}{2}h......(ii)\end{array}$$
In these type of questions, make factor of 24 and 36 and choose the common values which satisfy the above equations.
$$\begin{array}{rl}& \text{24 = 2,3,4,6,8,}\boxed{12}\\ & 36=3,4,9,\boxed{12}\end{array}$$
Choose the common factor i.e. Put this value in equation (i)
$$\begin{array}{rl}& \frac{24}{x-y}+\frac{36}{12}=6\\ & \frac{24}{x-y}+3=6\\ & x-y=8\\ & \therefore x+y=12\\ & \therefore x=10,y=2\\ & \text{Speed of the current,}\\ & y=2\text{ km/h}\end{array}$$