Discussion Forum

In a stream running at 2 kmph, a motorboat goes 6 km upstream and back again to the starting point in 33 minutes. Find the speed of the motorboat in still water.

Answer:

Explanation:

Let the speed of the motorboat in still water be $x$ kmph.

Then, speed downstream $=(x+2)$ kmph; Speed upstream $=(x-2)$ kmph.

$\therefore$ $\frac{6}{x+2}$ $+ \frac{6}{x-2}$ $=\frac{33}{60}$

$\Leftrightarrow$ $11x^{2}- 242x$ $+2x-44$ $=0$ $\Leftrightarrow$ $(x-22)(11x+2)=0$ $\Leftrightarrow x=22$. Hence, speed of

motorboat in still water = 22 kmph.