1. A. Monthly electric bill is normally distributed. The average monthly bill of
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Question
1.
A. Monthly electric bill is normally distributed. The average monthly bill of a random sample of 16 residents of a city is $130 with a standard deviation of $40. Construct a 90% confidence interval or the can monthly electric bills of all residents. Use exactly two decimal places for your answers.
B.) Annual income of electricans is normally distributed. A random sample of 25 electricians showed an average yearly income of $99,000 with a standard deviation of $20,000. Build a 95% confidence interval for the average yearly income of all electricians. No decimal place is required
Explanation / Answer
A) n=16 , mean is 130 and sd is 50
z for 90% confidence is 1.645
thus, the lower limit of confidence interval is 130-50*1.645/sqrt(16)=109.44
the upperlimit of confidence is 130+50*1.645/sqrt(16)=150.56
B) n is 25, mean is 99000 and sd is 20000
z for 95% confidence is 1.96
thus lower limit is 99000-20000*1.96/sqrt(25)=91160
upper limit is 99000+20000*1.96/sqrt(25)=106840
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