Stacy sells homemade pizzas, sizes large and small. A small pizza has 10 olive pieces and 18 mushroom pieces. A large pizza has 15 olive pieces and 45 mushroom pieces. She has enough olives and mushrooms for 60 olive pieces and 135 mushrooms pieces. Let x represent the number of small pizzas and y represent the number of large pizzas. Write all of the constraints for this situation, and explain each.