设x+y=2t (1)
y+z=3t(2)
z+x=4t(3)
则(1)+(2)+(3),x+y+z=9t/2(4)
(4)-(2),x=3t/2
(4)-(3),y=t/2
(4)-(1),z=5t/2
2x+4y+6z=3t+2t+15t=20t=120,得t=6
所以x=9,y=3,z=15