Question

A boat having a speed of 5 km/hr in still waters, crosses a river of width 1 km along the shortest possible path in 15 minutes. The speed of the river water in km/h is

• A. 1
• B. 3
• C. 4
• D. √41

Answer 1

Answer: (B)

3

Answer 2

Speed of boat v = 5 km/h

Speed of boat in flowing water v_b = 1/¼ = 4 km/h

Speed of the river v_r = $\sqrt{v^{2} - v_{b}^{2}}$ = $\sqrt{5^{2} - 4^{2}} = 3kmh^{- 1}$