Possible answers:
5
6
7
8
9
And the correct answer is...
6!!!
Note: Red circles are my answer, green arrows are some possible answers from some really "civilian" users xD (Sorry Jess)
This, though, is MY answer. You could have different LOGICAL ideas too.
(Like, Jess told me you can actually use the tank to store rubbish, and that makes 7. Yea right, Jess does that.)
