Question

The width of river is $1\, km$. The velocity of boat is $5 \,km/hr$. 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 29$ km / hr
• D. $ \sqrt 41$ km / hr

Answer 1

Answer: (A)

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