Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is normally distributed with true standard deviation 0.78. . Compute a 98% CI for the true average porosity of a certain seam if the average porosity for 25 specimens from the seam was 4.87. . Compute a 99% CI for true average porosity of another seam based on 18 specimens with a sample average porosity of 4.58. . How large a sample size is necessary if the width of the 98% in- terval is to be 0.40