It cannot be 5 and 7 as they are blocked.
Otherwise, 7 is filled up first.
It cannot be 1 because 1 leaks to 2 and 3. The leak stops the glass filling up fully.
It cannot be 2 and 3 because 2 leaks to 4 at 60% water mark and 3 leaks to 6 at 60% water mark.
3 will fill up faster than 2 by 1 because 3 gets liquid beer before 2.
Therefore, 6 gets filled first.
Followed by 4, 3, 2 and 1.