An atomic absorption method for the determination of the amount of iron present in used jet engine oil was found from pooling 30 triplicate analyses to have a standard deviation s = 3.6 mg Fe/mL. If s is a good estimate of u, calculate the 95 and 99% confidence intervals for the result 18.5 mg Fe/mL if it was based on
(a) A single analysis,
(b) The mean of two analyses,
(c) The mean of four analyses.