Note: Taking 8 kilograms more means taking 4 kilograms less is equal, so there is 8÷2. In the case of equality, it should actually be taken away: 12-8÷2.

At this time, the proportion of Party A shall be reduced: (5-3)÷2.

So each copy should be: 12-8÷2)÷[(5-3)÷2]

So a should be: (12-8÷2)÷[(5-3)÷2]×5=40.