Caitlin and Greg collect stamps. Each starts with a collection of 50 stamps. Caitlin adds 15 stamps to her collection each week. Greg adds 1 stamp to his collection the first week, 3 stamps the second week, 5 stamps the third week, and so on. Identify the function that gives the number of stamps in each collection as linear or nonlinear. Which collection is growing more quickly between week 5 and week 6?