CJ Computer Disks stocks and sells recordable CDs. The monthly demand for these CDs is closely approximated by a normal distribution with a mean of 20,000 disks and standard deviation of 4,000 disks. CJ receives shipments from the supplier once per month (at the beginning of each month).
 
a. Find the probability that the demand for recordable CDs exceeds 30,000 for a particular month.
b. Find the probability that the demand for recordable CDs is between 12,000 and 18,000.
c. How large an inventory must CJ have available at the beginning of the month so that the probability of running out of recordable CDs (a stock out) during the month is no more than .05?

Parker Seal, Inc. makes O-rings for newly planned commercial space shuttles. The rings are designed to seal connections and joint fillings in the fuel system to prevent leaks. One type of ring should be 5 centimeters in diameter in order to fit properly. A sample of 49 of these type of O-rings is selected and the diameters are measured  The sample results are as follows:
 
Sample Size = 49
Sample Mean = 5.05 centimeters.          
Sample Standard Deviation = .14 centimeters. 

a. Construct a 99% confidence interval for the average diameter of this type of O-Ring.
b. Interpret this interval.
c. How large a sample size will need to be selected if we wish to have a 99% confidence interval for the average diameter of this type of O-Ring with a margin for error of .02 centimeters?