Let speed of boat = x km/h

and speed of River Current = y km/h, (x > y)

Speed of boat in downstream = (x + y) km/h

Speed of boat in upstream = (x - y) km/h

Distance covered in downstream from A to B is = speed . time = 5(x + y) hours

Distance covered in upstream from B to A is = speed . time = 5(x - y) hours

Since, the distance is same,

5(x + y) = 7(x - y)

12y = 2x

Since the distance covered by raft from A to B is also same.

Distance covered by raft = 5(x + y)

Speed of raft = speed of current = y

Time taken by raft

hours.

It will take 35 hours by raft.