For 10 points. Why can you add 3x + 5x but not 3x + 5y?

The table below lists recommended amounts of food to order for 10 party guests. how much of each food item should nathan and amanda order for a graduation party with 55 guests? use the table to answer. item amount fried chicken 16 pieces lasagna 7 pounds deli meats 1.8 pounds sliced cheese 1 and two fifths pounds bakery buns 1 dozen potato salad 2 pounds