Final answer:
To find the speed of the boat in still water, subtract the speed of the stream from the total speed of the boat and stream when moving upstream.
Step-by-step explanation:
To find the speed of the boat in still water, we need to consider the relative motion of the boat and the stream. When the boat is moving upstream, it is going against the current, so its speed will be decreased. Let's assume the speed of the boat in still water is x mph. Since the speed of the stream is 3 mph, the speed of the boat when moving upstream will be (x - 3) mph.