Can you give me some step-by-step instructions for this math problem: Sidney has 4 kittens and 3 adult cats.  She has 7 cans of cat food.  Each adult cat eats 1 can of food per day.  Each kitten eats 3/4 of a can per day.  How many additional cans of food does Sidney need to buy to feed all of her animals for 7 days.