A swimmer can swim 68 km downstream and 45 km upstream in 9 hours and he will take 2 hours more to swim 51 km downstream and 72 km upstream. Find the speed of the swimmer in still water.

A man can row with the stream at 9 km/hr. and against the stream at 3 km/hr. His speed in still water is -

A motorboat whose speed is 20 km/h in still water takes 30 minutes more to go 24 km upstream than to cover the same distance downstream. If the speed of the boat in still water is increased by 2 km/h, then bow much time will it take to go 39 km downstream and 30 km upstream?