15. A store sells $A$ type exercise books, each retailing for 0.30 yuan, a dozen (12 books) for 3.00 yuan, and for purchases of more than 10 dozen, each dozen can be paid for at 2.70 yuan.
(1) Class 3(1) has a total of 57 students, each needing 1 $A$ type exercise book. If the class buys collectively, what is the minimum amount they need to pay in yuan?
(2) The third year of junior high school has a total of 227 students, each needing 1 $A$ type exercise book. If the grade buys collectively, what is the minimum amount they need to pay in yuan?
Three, 15. (1) Can buy 5 dozen or 4 dozen plus 9 books, the former requires a payment of $3.00 \times 5 = 15.00$ (yuan), the latter only requires a payment of $3.00 \times 4 + 0.3 \times 9 = 14.70$ (yuan). Therefore, when the class buys together, the minimum payment required is 14.70 yuan.
(2) $227 = 12 \times 18 + 11$, can buy 19 dozen or 18 dozen plus 11 books, the former requires a payment of $2.70 \times 19 = 51.30$ (yuan), the latter requires a payment of $2.70 \times 18 + 0.3 \times 11 = 51.90$ (yuan), which is 0.60 yuan more than the former. Therefore, when the grade buys together, the minimum payment required is 51.30 yuan.