1.2x+7y-z=24 ①

4x-4y+z=-3 ②

2.x+y+z=7

2x+y-z=5

x-y-2z=4

3.a+b+c=0

4a+2b+c=3

9a-3b+c=28

4.a+b+c=-2

a-b+c=20

a+b=0

5.4a+3b+0.5c=36,

a+b+c=36

6.x + z ＝ y

7z ＝ x + y + 2

x + y + z ＝14

7.2a+s+d=15 a+2s+d=16 a+s+2d=17

8.2a+7b=3 3a-c=1 -b+3c=4

9.2a=3s=6d a+2s+d=16

10.A:S:D=3:4:5 A+S+D=36

11.3x-y+z=3,2x+y-3z=11,x+y+z=12

12.5x-4y+4z=13,2x+7y-3z=19,3x+2y-z=18

13.a+b+c=-4,49a+7b+c=8,25a+5b+c=0

14.x+y/2=z+x/3=y+z/4,x+y+z=27

15.X=2Y,2X+Y+2Z=1,2X-Z=7,

16.Y-2Z=4,Y-3Z=10； 3X+Y=9.

17.x-2y-3z+18=0,x+3y-2z-8=0,x+y+2z-24=0

18.90a+30b+c=300,160a+40b+c=150,250a+50b+c=100

19.x:y=1:5,y:z=2:3,x+y+z=27

20.x+y=3,2x-y+2z=2,x-y-z=-3