A motorboat, whose speed in 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:

Explanation:

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

Speed downstream = 15 + $ x $ km/hr,

Speed upstream = 15 - $ x $ km/hr.

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

$\Rightarrow$ $\dfrac{900}{(225 - x^2)}$ = $\dfrac{9}{2} $

$\Rightarrow$ 9$ x $² = 225

$\Rightarrow x $² = 25

$\Rightarrow x $ = 5 km/hr.