7.21 Time spent using e-mail per session is normally distributed,
with J.L = 8 minutes and if = 2 minutes. If you
select a random sample of 25 sessions,
a. •vhat is the probability that the sample mean is between
7.8 and 8.2 minutes’?
b. what is the probability that the sample mean is betwe,’ll
7.5 and 8 minutes?
c. I f you select a random sample of 100 sessions, what is
the probability that the sample mean is between 7.8 and
8.2 minutes?
d. Explain the difference in the results of (a) and (c).

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