To determine the speeds (unit rates) of each student in miles per minute, we'll need to know the distance each student walked and the time it took them to complete that distance.
Since the graph is not visible, please provide the distances and times for Millie, Sam, and Zola, and I'll be happy to calculate their speeds for you. Once we have the speeds, we can also identify who walks the fastest, who walks the slowest, and who arrives at the finish line first.