In a random sample of 53 concrete specimens, the average porosity (in percent) was 21.6 and the standard

deviation was 3.2.

a. Find a 90% confidence interval for the mean

porosity of specimens of this type of concrete.

b. Find a 95% confidence interval for the mean

porosity of specimens of this type of concrete.

c. What is the confidence level of the interval

(21.0, 22.2)?

d. How many specimens must be sampled so that

a 90% confidence interval specifies the mean to

within±0.3?

e. How many specimens must be sampled so that

a 95% confidence interval specifies the mean to

within±0.3?

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