Variance of Zhou Xiao =15 [(70-80) 2+(80-80) 2+(80-80) 2+(90-80) 2+(80-80) 2] = 40;

So the answer is: 80, 40.

(2) Choose Zhou Xiao to participate in the competition;

Reason: Sun Xiao and Zhou Xiao have the same average score, but Zhou Xiao's score variance is smaller than Sun Xiao's, so Zhou Xiao's score is more stable, so he chose Zhou Xiao to participate in the math contest ... (6 points).