Question

A man can row three quarters of a kilometers against the stream in $11\frac{1}{4}$ minutes and return in $7\frac{1}{2}$ minutes.The speed of the man in still water is

• A. 2 km/hr
• B. 3 km/hr
• C. 4 km/hr
• D. 5 km/hr

Answer 1

Answer: (D)

5 km/hr

Let he speed of the man be $x$ km/hr and that of the water current be $y$ km/hr.

While going upstream, the relative speed of the man w.r.t stream is $x-y$ km/hr.

While going downstream, the relative speed of the man w.r.t stream is $x+y$ km/hr.

As per given condition,

$\frac{3/4}{x-y}=11\frac{1}{4}$ min and

$\frac{3/4}{x+y}=7\frac{1}{2}$ min

Dividing the above 2 equations,

$\frac{x+y}{x-y}$ = $\frac{3}{2}$

$\to 2(x+y)=3(x-y)$

$\to 5y=x$

Putting this value of $x$ in equation 2:

$\frac{3/4}{6x/5}=\frac{15}{120}$

$x=5$ km/hr