Lisandro swims for 2 hours in a stream that has a current of 4 mph. He leaves his dock and swims upstream for 3 miles and then back to his dock. What is his swimming speed in still water?

sample answer: you can determine whether each element of the domain is paired with exactly one element of the range. for example, if given a graph, you could use the vertical line test; if a vertical line intersects the graph more than once, then the relation that the graph represents is not a function.