(x+y)*z=1 (120%x+y)*0.9z=1 (x+0.75y)*(z+2.5)=1 三个方程联立,可能麻烦一点也可以

(x+y)*z=1 ①
(120%x+y)*0.9z=1 ②
(x+0.75y)*(z+2.5)=1 ③
①/②得(x+y)/[(1.2x+y)*0.9]=1,即x+y=1.08x+0.9y,即0.08x=0.1y,y=(4/5)x
代入①得(9/5)xz=1,xz=5/9
代入③得,(x+0.75*(4/5)x)*(z+2.5)=1
(8/5)x*(z+5/2)=1
(8/5)*(5/9)+(8/5)*(5/2)x=1
解得x=1/36
将x代入xz=5/9,得z=20