Ramu can row 9 km/hr in still water. It takes him twice the time to row up as to row down. Find the rate of flow of the stream.

Taking twice as long upstream means the upstream speed is half the downstream speed: 9 + s = 2(9 − s). So 9 + s = 18 − 2s, giving 3s = 9 and s = 3 km/h (check: downstream 12 km/h, upstream 6 km/h).