Question

David rowed a boat upstream for three miles and then returned to point he started from. The entire journey took four hours. The speed of the stream is one mile per hour. Find David's speed in still water. (Hint: speed = distance ÷ time, upstream speed = speed of the boat – speed of the stream, and downstream speed = speed of the boat + speed of the stream)

David's speed in still water is ____ miles per hour.