Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is normally distributed with true standard deviation 0.71. {a} Compute a 95% CI for the true average porosity of a certain seam if the average porosity for 23 specimens from the seam was 4.85. (Round your answers to two decimal piaces.) (:_E) (b) Compute a 98% CI for true average porosity of another seam based on 19 specimens with a sample average porosity of 4.56. (Round your answers to two decimal places.) {c} How large a sample size is necessary if the width ofd'le 95% interval is to be 0.33? {Round your answer up to the nearest whole number.) |:| specimens {d} What sample size is necessary to estimate true average porosity to within 0.25 with 99% condence? (Round your answer up to the nearest whole number.)