A boat which has a speed of `5 km// hr ` in steel water crosses a river of width ` 1 km` along the shortest possible path in `15 minutes`. The velocity of the river water in ` km// hr` is

Options

A. `1`
B. `3`
C. `4`
D. sqrt14`

Answer

B

Explanation

If ` v_R` is the resultant velocity of river water (v_r) and of boat (v_b), then `
` v_R= ( 1 km)/(15 min). = ( 1 km)/((1//4)h)= 4 km h^(-1)`
` v_r = sqrt(v_b^2 - v_R^2) = sqrt (5^2 - 4^2) = 3 km//h`
.