16. A man can row 9 km/hr in still water and he finds that it takes him thrice as much time to row up than as to row down the same distance in river. The speed of the current is :

17. A man can row three quarters of a kilometer against the stream in 11 minutes and return in 7 minutes. The speed of the main still water is :

18. A boat travels upstream from B to A and downstream from A to B in 3 hours. If the speed of the boat in still water is 9 km/hr and the speed of the current is 3 km/hr, the distance between A and B is :