Assume that the helium porosity of coal samples taken from any particular seam is Normally distributed with true standard deviation 0.75.
a. Compute a 95% CI for the true average porosity of a certain seam if the average porosity for 20 specimens from the seam was 4.85.
b. Compute a 98% CI for true average porosity of another seam based on 16 specimens with a sample average of 4.56.
c. How large a sample size is necessary if the width of the 95% interval is to be 0.40?
d. What sample size is necessary to estimate the true average porosity to within 0.2 with 99% confidence?