The speed of a boat is 5 km/hr is still water. It crosses a river of width 1 km along the shortest possible path in 15 minutes. The velocity of river water is :
(1) 3 km/hr
(2) 4 km/hr
(3) 5 km/hr
(4) 2 km/hr
Correct Answer is: (1) 3 km/hr
Vmr = 5 km/hr
t = 15 min
t = d/√(V2mr - V2r) ⇒ 15/60 = 1/√(25 - V2r)
⇒ 4 = √(25 - V2r) ⇒ V2r = 25 – 16 ⇒ V2r = 9
⇒ Vr = 3 km/hr
