Assume that the helium porosity (in percentage) of coal samples taken from any particular seam is normally distributed with true standard deviation0.76.

(a) Compute a 95% CI (use z-interval) for the true average porosity of a certain seam if the average porosity for19specimens from the seam was 4.85. (Round your answers to two decimal places.)

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(b) Compute a 98% CI (use z-interval) for true average porosity of another seam based on11specimens with a sample average porosity of 4.56. (Round your answers to two decimal places.)

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(c) How large a sample size is necessary if the width of the 95% interval is to be0.38? (Round your answer up to the nearest whole number.)

specimens

(d) What sample size is necessary to estimate true average porosity to within0.23with 99% confidence? (Round your answer up to the nearest whole number.)

specimens