How large a sample should be selected to provide a 95 confid

How large a sample should be selected to provide a 95% confidence interval with a margin of error of 3? Assume that the population standard deviation is 20.

Compute Sample Size
n = (Z a/2 * S.D / ME ) ^2
Z/2 at 0.05% LOS is = 1.96 ( From Standard Normal Table )
Standard Deviation ( S.D) = 20
ME =3
n = ( 1.96*20/3) ^2
= (39.2/3 ) ^2
= 170.738 ~ 171