Question

A motor boat whose speed is 15 km/hr in still water goes 40 km downstream and comes back in a total of 6 hours. The speed of the stream is

• A. 9 km/hr
• B. 7 km/hr
• C. 5 km/hr
• D. 3 km/hr

Answer 1

The correct option is C 5 km/hr

Let speed of the stream be x km/hr
Speed downstream $=(15+x)km/hr$
and speed upstream $=(15-x)km/hr$
$\frac{40}{15+x}+\frac{40}{15-x}=6$
$\Rightarrow \frac{40(15-x+15+x)}{(15+x)(15-x)}=6$
$\Rightarrow 225-x^2=200$
$\Rightarrow x^2=25$
$\therefore x=-5or+5$

speed cannot be negative
Thus, the speed of the stream =$5km/hr$