hi joyfrancis

i would like to give u a very short method for the first question.see the soln. given by indian army is correct but the method is long as he has done it from ground frame.

now see if we go and sit in the raft the velocity of river w.r.t us will become zero

and the velocity of ground w.r.t to us will become negative of the velocity of river w.r.t to ground.now see the figure . the time taken by motorboat to go and cme back is 2hrs.

now in these two hrs the tree moves 6 km toward left.

therefore velocity of tree =velocity of ground = 6/2 = 3km/hr towards left

therefore velocity of river will become 3km/hr towards right(see bold part).

hence the ans. is 3km/hr.DO SEE THE FIG> FOR CLARITY CLICK TO ENLARGE