Question

A boat which has a speed of 5 kmh^-1 in still waters crosses a rivers of width 1 km along the shortest possible path in 15 minutes. The speed of the river in kmh^-1 is

• A. 1
• B. 3
• C. 4
• D. √41

Answer 1

Answer: (B)

3

Answer 2

Speed of boat, v_b = 5 kmh^-1

Speed of boat when water flows, vr = $\frac{1}{1/4}$ = 4 kmh^-1

Resultant speed v = $\sqrt{vb^{2} - vr^{2}} = \sqrt{5^{2} - 4^{2}} = \sqrt{25 - 16} = \sqrt{9}$