x+y+z=51 4x+8y+5z=300 x+y+2z=67

x+y+z=51 (1)
4x+8y+5z=300 (2)
x+y+2z=67 (3)
由(1)得:x+y=51-z 代入(3)得
51-z+2z=67
z=67-51=16
则
x+y=51-16=35 (4)
4x+8y+5*16=300 =>4x+8y=220 =>x+2y=55 (5)
联立(4)(5)解得
x=15
y=20
所以
x=15
y=20
z=16

x+y+z=51 ①
4.5+9y+7.5z=360 ②
4x+8y+5z=300 ③
9*①-2*②得 -9y-6z=-261 ④
4*①-③得 -4y-z=-96 ⑤
9*⑤-4*④得 15z=180,z=12
代入z到④,得y=21
代入y和z到①,得x=18
因此 x=18 y=21 z=12
o(∩_∩)o

x+y+z=5 ① 4x+8y+5z=300 ② x+y+2z=67 ③
③-①,z=62
代入①、②,x+y=-57 ④,4x+8y=-10 即x+2y=-2.5 ⑤
⑤-④,y=54.5
代入④,x=-111.5