1.90%x（1+20%）+90%x（1+15%）*（2200-x）=2200+1312.12%x+14x%（30-x）=43.90%x*24（1+20%x）=x

1.90%x（1+20%）+90%x（1+15%）*（2200-x）=2200+131
0.9x(1.2+1.15*(2200-x))=2200+131
0.9x(1.2+2530-1.15x)=2331
1.08x+2277x-1.035x=2331
2277.045x=2331
x约等于1.0236

1.
90%x(1+20%)+90%x(1+15%)*(2200-x)=2200+131
90%x[(1+20%)+(1+15%)*(2200-x)]=2331
90%x[120%+115%*(2200-x)]=2331
90%x(120%+2530-115%x)=2331
90%x(5%+2530)=2331
90%x(5%+2530)=2331
0.045x+2277=2331
0.045x=54
45x=54000
x=1200
2.
12%x+14x%(30-x)=4
12x+14x(30-x)=400
12x+420x-14x^2=400
432x-14x^2-400=0
216x-7x^2-200=0
7x^2-216x+200=0
x1=(108+2根号2566)/7,x2=(108-2根号2566)/7
3.
90%x*24(1+20%x)=x
2160%x*(1+20%x)=x
21.6x+4.32x^2=x
2160x+432x^2=100x
2160x+432x^2-100x=0
432x^2+2060x=0
108x^2+515x=0
x(108x+515)=0
x=0
108x+515=0
x1=0,x2=-515/108