SOLUTION: The principal at Riverside highschool would like to estimate mean length of the time each day that it takes all of the busses to arrive and unload the students. How large of a samp

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Question 1147998: The principal at Riverside highschool would like to estimate mean length of the time each day that it takes all of the busses to arrive and unload the students. How large of a sample is needed if the principal would like to assert with 90% confidence that the sample mean is off by, at most, 7 minutes. Assume that s=14 minutes based on previous studies.
Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
The half-interval is 7 minutes and is t (df=unknown)*14/sqrt(n)
use z first to get an idea
1.645*14/sqrt(n)=7
23.03/sqrt(n)=7
sqrt(n)=3.29
n=10.82
try t=12 where df=11 and t=1.796
1.796*14/sqrt(12)=7.26
try t=13 where df=12 and t=1.782
1.796*14/sqrt(13)=6.97
sample size of 13