设买苹果x Kg,橘子y Kg,梨z Kg.
x + y + z = 30 ①
4x + 3y + 2z =80 ②
由②-①*2得 y = 20 - 2x ③
由③代入①得 z = 10 + x ④
∵0 ＜ y ＜ 30
∴0 ＜ 20 - 2x ＜ 30
∴-5 ＜ x ＜ 10
∵x ＞ 0
∴0 ＜ x ＜ 10
① x = 1,y = 18,z = 11
② x = 2,y = 16,z = 12
③ x = 3,y = 14,z = 13
④ x = 4,y = 12,z = 14
⑤ x = 5,y = 10,z = 15
⑥ x = 6,y = 8,z = 16
⑦ x = 7,y = 6,z = 17
⑧ x = 8,y = 4,z = 18
⑨ x = 9,y = 2,z = 19
共以上九种情况.