Question 393634
Let X = r.v. representing the # of defects.

Then mean is {{{mu = E(X) = np = 500*0.09  =45}}}, variance is 

{{{sigma^2 = npq = 500*0.09*0.91  =40.95}}}, and standard deviation is

{{{sigma = sqrt(40.95) = (3sqrt(455))/10  =6.4}}} to the nearest tenth.