621
xkcd #2922: Pub Trivia
(imgs.xkcd.com)
Alt text:
Bonus question: Where is London located? (a) The British Isles (b) Great Britain and Northern Ireland (c) The UK (d) Europe (or 'the EU') (e) Greater London
~~Then wouldn't 4 fail the conjecture? 4 = 1+ 3 but 1 is not a prime.~~
Nevermind my sleepy self somehow thought 2 isn't a prime number :)