A person is ordered by doctor to take 12 units of vitamin A, 9 units of vitamin D, and 7 units of vitamin E each day. There are three brands of vitamin supplement available: brand X contains 2, 1, and 2 units, brand Y contains 3, 2, and 2 units and brand Z contains 4, 4, and 1 units of vitamin A, D, and E, respectively. How many pills of each brand will provide the required amounts of vitamin? pills of brand X, pills of brand Y and pills of brand Z.