X-1000=X*10%+800
X-X*10%=1000+800
(1-10%)*X=1800
90%*X=1800
9/10 *X=1800
X=1800*10/9
X=200090%能来个说明吗还有（1-10%）1=100% 100%-10%=90%1*X=100%X =X