Salma purchased a prepaid phone card for $15 Long distance calls cost 6 cents a minute using this card. Salma used her card only
1 answer:
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Answer:
a) [24.114,29.2858]
b) Since 30 is not in the 95% confidence interval, there is a 95% probability that 30 is not the true mean WEP
Step-by-step explanation:
a)
The 95% confidence interval is given by the interval
where
= 26.7 is the sample mean
s = 17.7 is the sample standard deviation
n = 180 is the sample size
Since the sample size is big enough, we can use the Normal N(0,1) to compute and it would be 1.96(*) (a value such that the area under the Normal curve outside the interval [-z, z] is 5% (0.05))
and our 95% confidence interval is
(*)
This value can be computed in Excel with
<em>NORMINV(1-0.025,0,1)</em>
and in OpenOffice Calc with
<em>NORMINV(1-0.025;0;1)</em>
b)
Since 30 is not in the 95% confidence interval, there is a 95% probability that 30 is not the true mean WEP