Discussion Forum

A motorboat, whose speed is 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is :

Answer:

Explanation:

Let the speed of the stream be $x$ km/hr.Then,

Speed downstream $=(15+x)$ km/hr, Speed upstream $=(15-x)$ km/hr

$\frac{30}{(15+x)}+\frac{30}{(15-x )}$ $=4\frac{1}{2}$  

$\Leftrightarrow \frac{900}{(225-x^{2})}$ $=\frac{9}{2}$

$\Leftrightarrow 9x^{2}$ $=225$

$x^{2}$ $=25$ $\Leftrightarrow x$ = 5 km/hr.