Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is normallv distributed with true standard deviation 0.73. [a] Compute a 95% CI for the true average porosity of a certain seam if the average porosity for 21 specimens from the seam was 4.85. (Round your answers to two decimal places.) _./ .J ( ) (b) Compute a 98% CI For true average porosity of another seam based on 14 specimens with a sample average porosity of 4.55. (Round vour answers to two decimal places.) (3: ._x) (c) How large a sample size is necessary if the width of the 95% interval is to be 0.47? (Round your answer up to the nearest whole number.) (cl) What sample size is necessary to estimate true average porositv to within 0.25 with 99% confidence? [Round your answer up to the nearest whole number.)