Question 1027426
Let X = number of students in a class

==> {{{E(X) = mu = (1*30 + 8*60 + 1*70 + 4*100)/14 = 980/14 = 70}}}

{{{Var(X) = (1/14)sum(f[i]*(x[i]-mu)^2, i=1, 4) = (1/14)(1*(30-70)^2 + 8*(60-70)^2 + 1*(70-70)^2 + 4*(100-70)^2) = 3000/7}}}

==> {{{SD(X) = sqrt(Var(X)) = sqrt(3000/7) = 20.7 }}}