Courtney visited her grandmother. Her grandmother used to collect stamps. She had a shoebox full of 5 cent, 6 cent, and 7 cent stamps. Courtney thought, "I could mail a lot of different-sized letters and postcards with
these stamps." She tried to figure out the different amounts of postage she could make with a combination of
those stamps. What totals could she make using no more than three stamps, and what totals are impossible?