9a laboratory tested 72 chicken eggs and found that the mean amount of cholesterol was 240 milligrams with s=19.6 milligrams. construct a 95 percent confidence interval for the true mean cholesterol content, μ, of all such eggs.

Note that we're dealing with a sample here, specifically, 72 sample chicken eggs.  Sample mean is 240 mg.  Want 95% confidence interval.

Use the following formula for the 95% confidence interval of a sample:
                                                             sample std. dev.
sample mean  plus or minus  (1.602)
                                                            sqrt(n)
                                                                                       19.6 mg
Here, this gives us      240 mg plus or minus (1.602)
                                                                                     sqrt(72)

which simplifies to        240 mg plus or minus (1.602) (2.310), or

                                      240 mg plus or minus 3.7 mg

the 95th confidence interval for the true mean cholest. content is 
 (-236.3, 243.7).

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