20%＋（1240-x）×30%=1240×25%

解方程：

（1240-x）×30%=1240×25%-20%

1240×30%-30%x=1240×25%-20%

30%x=1240×30%-1240×25%-20%

x=（1240×30%-1240×25%-20%）/30%

  = {1240(30%-25%)-20%}/30%

  = (1240x5%-20%）/30%

  =（62-20%）/30%

  =62x100/30-20%x100/30

  =6200-20

  =6000

解之得：x=6000

20%+(1240-x)*30%=1240*25%

0.2+372-0.3x=310

0.3x=372.2-310

0.3x=62.2

x=62.2/0.3

x=622/3