The speed of boat is `5 kmh^(-1)` in still water. It crosses a river of width 1 km along the shortest possible path in 15 min. Then, velocity of river will be

Options

A. 4 `"km"//"hr"`
B. 3 `"km"//"hr"`
C. 2 `"km"//"hr"`
D. 1 `"km"//"hr"`

Answer

B

Explanation

15 min = "1/4" hr.

`(d)/(V_(y))=(1)/(4)implies (1)/sqrt(V_(MR)^(2)-V_(R)^(2))=(1)/(4)`=`(1)/sqrt(5^(2)-V_(R)^(2))`implies`V_(R)= 3 "km/h"`