Question

A motor boat going downstream overcomes a float at a point a 60 minutes later it turns and after some time passes the food at a distance of 12 km

Answer 1

Let V(f) = velocity of flow & V = velocity of boat

Distance covered by boat in 60 mins = (V + V(f)) x 1 hour = V + V(f)

Also
6 km = V(f )* (60 min + T) where T is time taken for the boat to return to the bottle again.
6 = Vf [1 hour + (V + Vf - 6) / (V - Vf) ] ........... V - V(f) is net speed for upstream motion
6 = {V(f).(2V - 6)}/ V - Vf
6V - 6V(f) = 2V.V(f) - 6V(f)
6V = 2V.V(f)
V(f) = 3

So the flow speed = 3 km/hr