E= Za/2 * (population standard deviation/ sqrt sample size) So we have a score for 95% confidence level is 1.96 (Za/2= 1.96) (Standard deviation= 30.86) (Sample size= 40) Put these into the formula from above 1.96*(30.86/sqrt 40)= 9.56 E=9.56 Finally,we do E- sample mean < u < E+sample mean Sample mean= 146.22 146.22-9.56=136.66 146.22+9.56=155.78 136.66