Question

A river runs at $2$ km/hr. If a man takes twice as long to row up the river as to row down, the speed of the man in still water is :

• A. $5$ km/hr
• B. $4$ km/hr
• C. $8$ km/hr
• D. $6$ km/hr

Answer 1

The correct option is D $6$ km/hr

Let the man’s rate upstream $‘b^{\prime }=x$ km/hr.

Then his rate downstream $‘a^{\prime }=2x$ km/hr

Speed of the stream$=2$ km/hr

$=\frac{1}{2}(a-b)=2$

$=(a-b)=4$

then,

$2x-x=4$

$x=4$

then $a=8$ km/hr and $b=4$ km/hr

Speed in still water$=\frac{1}{2}(a+b)=\frac{1}{2}(8+4)=6km/hr$