Question

The width of river is $1km$. The velocity of boat is $5km/hr$. The boat covered the width of river in shortest time $15\min$. Then the velocity of river stream is

• A. 3 km / hr
• B. 4 km / hr
• C. $\sqrt{2}9$ km / hr
• D. $\sqrt{4}1$ km / hr

Answer 1

Answer: (A)

$t=\frac{d}{\sqrt{u^2-v^2}}$
$\frac{1}{4}=\frac{1}{\sqrt{(5{)}^2-v^2}}$
$\frac{1}{16}=\frac{1}{25-v^2}$
$v=3km/hr$