A boat which has a speed of `5 km` per hour in still water crosses a river of width 1 km along the shortest possible path in fifteen minutes. The velocity of the river water in km per hour is :-

Options

A. 1
B. 3
C. 4
D. `sqrt(41)`

Answer

B

Explanation

`vecV_(br)=vecV_(b)-vecV_(r)`
`V_(br)^(2)=V_(b)^(2)+V_(r)^(2)`
`V_(b)^(2)=25-V_(r)^(2) " " ...(1)`
`t=("distance")/(V_(b))rArr `
`(1)/(4) hr = (1)/(sqrt(25-V_(r)^(2)))`
`V_(r)=3 km//h`