See the concept of upstream downstream.
and try to do this sum.
i) A steamer goes downstream and covers the distance between two ports in 3 hours while it covers the same distance upstream in 4 hours. if the speed of the stream is 3 km/hr, find the speed of the steamer in still water.
refer this video too https://youtu.be/hso_yBVlskY . and bring NB1 for exam tomorrow.