1，2，3三个城市每年需分别供应电力320，250和350单位，由Ⅰ，Ⅱ两个电站提供，它们的最大可供电量分别为400个单位和450个单位，单位费用（元）如下表所示。由于需要量大于可供量，决定城市1的供应量可减少0单位～30单位，城市2的供应量不变，城市3的供应量不能少于270单位，试求总费用最低的分配方案（将可供电量用完）。(1, 2, 3, three city each year to 320250 and 350 respectively, power supply unit, an I, II provides two power stations, the maximum power supply which is 400 units and 450 units respectively, the unit cost (yuan) as shown in the following table. Because the demand is greater than the supply available, decided to supply 1 of the city can be reduced by 0 units to 30 units, the supply of city 2 unchanged, the supply is not less than 270 of the city 3 units, try to seek the lowest total cost allocation scheme (the power supply will be exhausted).)