This involves the concept of relativity and as a matter of fact, it might be wise to concentrate the problem without outside distractions.
While rowing his boat upstream, Anik drops a cork overboard and continues rowing for 10 more minutes. He then turns around, chasing the cork, and retrieves it when the cork has traveled 1 mile downstream. What is the rate of the stream?
Rather than approaching this problem by the traditional methods, common in an algebra course, consider the following:
It does not matter if the stream is moving and carrying Anik downstream, or is still. We are concerned only with the separation and the coming together of Anik and the cork. If the stream were stationary, Anik would require as much time rowing to the cork as he did rowing away from the cork.
That is, he would require 10 + 10 = 20 minutes. Since the cork travels 1 mile during these 20 minutes, its (i.e., the stream’s) rate of speed is 3 miles per hour.
It is a concept worth understanding, for it has many useful applications in everyday life thinking processes.
This is, after all, one of the purposes for learning mathematics.