Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is normally distributed with true standard deviation 0.74. (a) Compute a 95% CI for the true average porosity of a certain seam if the average porosity for 18 specimens from the seam was 4.85. (Round your answers to two decimal places.) , (b) Compute a 98% CI for true average porosity of another seam based on 18 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 of the 95% interval is to be 0.39