A boat can travel at a speed of 8km/h in still water on a lake.In flowing water of a stream, it can move at 8km/h relative to the water in the stream.If the stream speed is 3km/h, how fast can the boat travel upstream and downstream?

Answer 1

Upstream? the difference in velocities, downstream the sum.

Does that make sense?

vbg=vbs+vsg
where vbg is velocity of boat relative to ground; vbs is veloicty of boat relative to stream, and vsg is velocity of stream relative to ground. But note, in the case of upstream, the velocity of the stream relative to ground is negative, so one is dealing with the difference of the velocities.