Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is normally distributed with a true standard deviation of 0.75. (a) Compute a 95% confidence interval for the mean porosity of a certain seam based on 20 specimens with a sample mean porosity of 4.85.(b) How large a sample size is necessary if the width of a 95% interval is to be 0.40?