Question

The width of river is 1 km. The velocity of boat is 5 km/hr. The boat covered the width of river with shortest will possible path in 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)

$3 \ km/hr$

Answer 2

$t=\frac {d}{\sqrt {υ^2-v^2}}$

$\frac 14= \frac {1}{\sqrt {(5)^2-v^2}}$

$\frac {1}{16} = \frac {1}{25-v^2}$

$25-v^2 = 16$

$v^2 = 9$

$v =\sqrt {9}$

$v=3\ km/hr$

So, the correct option is (A): $3\ km/hr$