In water, the direction along the stream is called downstream. And, the direction
against the stream is called upstream.

If the speed of a boat in still water is u km / hr and the speed of the stream is
y km /hr, then, : Speed downstream = (u + y) km/ hr Speed upstream = (u - y) km/hr.

If the speed downstream is a km/hr and the speed upstream is b km /hr, then : Speed in still water = ^{1}/_{2} (a + b) km/hr Rate of stream = ^{1}/_{2} (a - b) km/hr