Kelly and Greta are asked to write an equation for the scenario below. "One person was able to plant 4 trees in the same amount of time that 5 people working together were able to plant 20 trees." The girls agree that x represents the number of people working and y represents the number of trees planted. Kelly wrote the equation y = one-fourth x, and Greta wrote the equation y = 4 x. Which person is correct, and why?