Determine the minimum sample size needed to estimate the mean seek time of 
disk drives from a production run to a level of accuracy of 0.025ms with 
99% confidence.
Assume that s(y) is known to be 0.075 from a previous study.

Solution:

Since the required z value for a 99% CI is 2.6 and you know from CI
theory that the 99% plus/minus amount for your estimate is:
           delta= z(99)*(s(y)/sqrt(n)) 
where delta is the level of accuracy then:
	   0.025=2.6*(s(y)/sqrt(n))
Rearranging terms:
	n=((2.6)*(0.075)/0.025)**2=60.84

Hence 61 disk drives would be required.