2 (2 (2a-1)-1)-1= 0 (2a-1), where 2a means twice as much as before, and-1means having a drink. It follows this reasoning in turn. a=7/8。

A is the original pot of wine, and 2a is because it has doubled, that is, A+A,-1 has reduced one barrel. 2a- 1 is the first wine, because it will be doubled the second time, that is, 2(2a- 1), and it will also reduce one barrel. (2(2a- 1)- 1) is the second wine, because it will double to 2(2(2a- 1)- 1) for the third time, and it will also reduce one barrel, 2(2(2a- 1).