Puzzle of the Day, page-16

All tubes are not blocked. 6 is completely open.

as 4 and 5 are blocked, they will fill until the very first opportunity to overflow (Water takes the easiest path of travel).

Therefore, the first opportunity is to overflow into 6, way before it can back-up to tube 2. There is not enough back pressure to allow any water back-up to 2.


By logic, 6 will fill first.