(60-50+x)(800-100x)=12000
(10-x)(8-x)×100=12000
(10-x)(8-x)=120
80-8x-10x+x²=120
x²-18x-40=0
(x-20)(x+2)=0
x-20=0 或者 x+2=0
x=20 或者x=-2