Venkat can row a boat in still water at the speed of $\displaystyle 12$ km/h. He ferries tourists $\displaystyle 15$ km upstream and $\displaystyle 18$ km downstream in $\displaystyle 3$ hours. Find the speed of the stream.

Let speed of the stream be x km/h \[\therefore \frac{15}{12-x}+\frac{18}{12+x}=3 \] \[\Rightarrow x^{2}-x-12=0 \] \[\Rightarrow(x-4)(x+3)=0 \] \[\Rightarrow x=4,-3 \] \[x=-3 \text { (rejected) } \] \[\therefore x=4 \] The speed of the stream \(\displaystyle =4 \mathrm{~km} / \mathrm{h}\)