Boat and Stream problem are depends on concept of Speed, Time and Distance.
If the speed of the water in the river is 0 then it is called as Still water.
It is denoted as Bs.
If the water of a river is flowing at constant speed then it is called as stream.
It is denoted as Cs.
In a river if a boat direction along the stream then it is called as Downstream.
It is denoted as Ds.
Down stream= still water+stream
In a river if a boat against the stream then it is called as Up stream.
It is denoted as Us.
Up stream= Still water - Stream
If the speed of a boat in still water is x km/hr and the speed of the stream is y km/hr, then
speed down stream=(x + y)km/hr
Speed Up stream= (x - y)km/hr
Bs=(Ds + Us)/2