A plus

Zach (doing his design and tech homework): how do I find where the middle is on this A3 sheet?
J: just measure in 21cms
Zach (laughing his head off) : OMG that's so nerdy - how on earth do you just know that without looking it up?
J (laughing also): I do a lot of work in photoshop - I can tell you the measurements of all A size paper - A4 is 29.7cms by 21cms, A3 is 29.7 by 42!'
Zach (still giggling hysterically): That's so sad ...
J: I know