x/28+y/35+z/30=4.5 ①x/35+y/28+z/30=4.7 ②x+y+z=142 ③①-②得(x-y)/140 = - 0.2x - y = - 28 ④①+②得9/140 (x+y) + z/15 = 9.2同乘以1527/28 (x+y) + z = 138 ⑤③-⑤得(x+y)/28 = 2x+y = 56 ⑥④+⑥得 2x = 2...