"Round to the nearest even number". "Round 'til even" doesn't work (also note the apostrophe; 'til is an abbreviation of until and usually said rather than written. If you want them to specifically round to the nearest higher number you would say "round up to the nearest even number" I've also normally seen "dividable by two" written as "divisible by two", but maybe it's an American/English variation (I'm English).

Haha, don't panic Michael. I'm not offended in the slightest. I'm always more than happy to be corrected when I'm wrong. Today I learnt that "till" is a correct usage :) Mood is so hard to read in text isn't it? If you see me make another mistake (this was obviously the only one I've ever made and I doubt it will happen ever again), please feel free to contradict me. I would much rather know I was wrong. I appreciate you clearing things up, but no slight was taken, I assure you :)

Duncan and Michael, thank you for your comments. As understand, neither till nor until fit in the phrase anyways. So I will use "Round to the nearest even number", as by Duncan's advice. I need it to be short and rather formal, because this is for instruction with formulae.

Duncan, I don't disagree! I just felt that Tatiana should know that she wasn't strictly "wrong" with her use of "till", which she has very likely seen in older texts, or maybe even been taught. I hope you're ok with that - no slight was intended :)

I stand corrected. Michael is right. However, If I see "till" I, personally, think of very informal writing among friends. I don't think I would use it when writing formal instructions - although I wouldn't use 'til either.