(1)
假设 竖纸盒 x 个,横纸盒 (100 - x) 个
正方形纸：竖纸盒 = x, 横纸盒 = 2 * (100 - x),一共 x + 2(100 - x) = 200 - x
长方形纸：竖纸盒 = 4x, 横纸盒 = 3 * (100 - x),一共 4x + 3(100-x) = 300 + x
(2)
正方形纸：200 - x 38 342 < 5y < 358
=> 68.4 < y < 71.6
y = 69, 70, 71
=> (x, y) = (24, 69), (22, 70), (20, 71)
(24, 69)：4x+3y = 96 + 207 = 303
(22, 70)：4x+3y = 88 + 210 = 298
(20, 71)：4x+3y = 80 + 213 = 293
因此 a = 293, 298, 303