Courtney visited her grandmother . Her grandmother used to collect stamps. She had a shoebox full of 5 cents, 6 cents and 7 cents stamps . Courtney thought , "I could mail a lot of different size letters and postcards with these stamps ." She tried to figure out the different amount of postage she could make with a combination of those stamps . What totals could she make and what totals are impossible ? Explain how you found your solution How do you know your solution is correct ?