(1) According to the meaning of the question, a=0.04×5× 1000=200, and b = 0.02× 5×1000 =100.

(2) Let the number of students with excellent grades be X, then X40 = 350+300+1001000, and the solution is: x=30.

That is, the number of students with excellent grades is 30.

(3) According to the meaning of the question, the value of X is 0, 1, 2, then

P(X=0)=C2 10C240=352，P(X = 1)= c 10c 130 c 240 = 5 13，P(X=2)=C230C240=2952

So the distribution list of x is

x？ 0? 1? 22? p？ 352? ? 5 13 2952? EX = 0×352+ 1×5 13+2×2952 = 32。