A boat crosses a river of width 1 km by shortest path in 15 min. if the speed of boat in still water is 5 `kmh^(-1)`, then what is the speed of the river?

Options

A. 5 `kmh^(-1)`
B. 12 `kmh^(-1)`
C. 3 `kmh^(-1)`
D. 4 `kmh^(-1)`

Answer

C

Explanation

let `v_(r)` be the belocity of river,
velocity of boat when crosses river along the shortest path
`v=(1km)/(15" min")=(1000)/(15xx60)xx(18)/(5)kmh^(-1)`
`=4kmh^(-1)`
therefore, speed of river
`v_(r)=sqrt(5^(2)-4^(2))`
`=3kmh^(-1)`
Durig motion, boat directs itself parallel to AB, moves along AC due to the resultant velocity.