a:低年级 b:中年级 c:高年级a + b = 120(1)b + c = 168(2)(2)-(1)得c - a = 48(3)a / c = 3 / 7 即 3c = 7a(3) * 7得7c - 7a = 48 * 7 = 336故4c = 336，c = 84
高年级作品有84件
