How do I add data to a new row and to a previous row?

[eric_niedergang]

Hi everybody,

I am looking for the best way to do this :
I want to fill in a form with datas from the sale of a newspaper
One field represents the newspapers I bring to a vendor
Another field represents the unsold newspapers from the previous issue that I take back
So the number of unsold newspapers should fill the previous row

How to do this as implicitly as possible?

Thanks