Camille's favorite book is about Johnny Appleseed, the American pioneer who planted apple trees all across the country. Inspired by the story, Camille plants an apple seed in her backyard and tends the seed as it slowly grows into a tree. There is a proportional relationship between the age of Camille's apple tree (in years), x, and the height of the tree (in feet), y. The equation that models this relationship is y=1.25x. If this rate continues, how old will Camille's apple tree be when it is 20 feet tall? Write your answer as a whole number or decimal.