Problem 4.6. Fourth-grader Vasya goes to the cafeteria every school day and buys either 9 marshmallows, or 2 meat pies, or 4 marshmallows and 1 meat pie. Sometimes Vasya is so busy socializing with classmates that he doesn't buy anything at all. Over 15 school days, Vasya bought 30 marshmallows and 9 meat pies. How many of those days did he not buy anything?