Question

A man rows upstream a distance of $9km$ or downstream a distance of $18km$ taking $3$ hours each time. The speed of the boat in still water is

• A. $7\frac{1}{2}km/h$
• B. $6\frac{1}{2}km/h$
• C. $5\frac{1}{2}km/h$
• D. $4\frac{1}{2}km/h$

Answer 1

The correct option is D $4\frac{1}{2}km/h$

Let the speed of a boat in still water be $x$ and speed of stream is $y$

Then, speed of boat in downstream is $x+y$

Speed of boat in upstream $=x-y$

Now,

$x+y=\frac{18}{3}$

$x+y=6$ ---- (i)

and

$x-y=\frac{9}{3}$

$x-y=3$ ----- (ii)

Add (i) and (ii)

$x=\frac{9}{2}Km/hr$

the speed of boat in still water is $4\frac{1}{2}Km/hr$