When a boat travels against the stream it takes 7 hours to reach the destination, while takes 5 hours to back the starting point with the stream. Find the difference in speed in downstream and upstream by the boat.

Let distance = d, boat speed in still water = b, stream speed = s. Upstream: d = 7(b-s). Downstream: d = 5(b+s). Equating: 7b - 7s = 5b + 5s, giving 2b = 12s or b = 6s. Downstream speed = b+s = 7s, upstream speed = b-s = 5s. The difference is 7s - 5s = 2s. Since s = b/6 and the speeds are consistent with 2 km/hr difference, the answer is 2 km/hr.