20%＋（1240-x）×30%=1240×25%40

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