Test runs for the operating time before overeating of 9 randomly selected motors of a certain...

Test runs for the operating time before overeating of 9 randomly selected motors of a certain type showed a sample mean of 39.7 hours and a sample standard deviation of 2.5 hours. Find a 95% confidence interval for the standard deviation of the length of time a motor of this type will operate before overheating.

Expert Solution

We have given here,


Sample standard deviation=2.5 hours
Sample size =9

Degree of freedom =n-1=   8
Level of significance given =1-0.95=0.05
Chi square critical value for lower tail=2.18

Chi square critical value for upper tail=17.535

We can use here confidence interval formula for population standard deviation

=(1.69,4.79)






Lower confidence limit for population standard deviation=1.69 hours

Upper confidence limit for Population standard deviation=4.79 hours

orchestra answered 1 year ago

certain flight arrives on time 85 percent of the time. Suppose 127 flights are randomly selected....

certain flight arrives on time 85 percent of the time. Suppose 127 flights are randomly selected. Use the normal approximation to the binomial to approximate the probability that (a) exactly 120 flights are on time. (b) at least 120 flights are on time. (c) fewer than 100 flights are on time. (d) between 100 and 120, inclusive are on time.

The time it takes a randomly selected rat of a certain subspecies to find its way...

The time it takes a randomly selected rat of a certain subspecies to find its way through a maze is normally distributed with µ = 1.5 minutes and σ = 0.35 minutes. Suppose 30 rats are selected. Let X1,X2,...,X30 denote their times in the maze. (a) What are the mean and standard deviation for total time for all 30 rats in the maze? (b) What is the probability that the total time for the 30 rats is between 45 and...

The time it takes a randomly selected job applicant to perform a certain task has a...

The time it takes a randomly selected job applicant to perform a certain task has a distribution that can be approximated by a normal distribution with a mean of 120 seconds and a standard deviation of 20 seconds. The fastest 10% are to be given advanced training. What task time qualifies individuals for such training? (Remember the fastest times takes the least amount of time.) Use Excel to find your answer .Round your answer to the nearest second.

30 students were randomly selected from a large group of students taking a certain test. The...

30 students were randomly selected from a large group of students taking a certain test. The mean score for the students in the sample was 86. Assume that σ = 11.1. Construct a 99% confidence interval for the mean score, μ, of all students

Test scores of 9 randomly selected students are: 83, 73, 62, 63, 71, 77, 59, 92,77...

Test scores of 9 randomly selected students are: 83, 73, 62, 63, 71, 77, 59, 92,77 Compute the 99% confident interval of the true mean.

A fan is designed to last for a certain time before it will have to be...

A fan is designed to last for a certain time before it will have to be replaced (planned obsolescence). The fan only has one speed (at a maximum of 675 rpm), and it reaches the speed in 3.0 s (starting from rest). It takes the fan 7.0 s for the blade to stop once it is turned off. The manufacturer specifies that the fan will operate up to 1 billion rotations. Andre lives in a hot climate, works outside of...

Calculate the test statistic F to test the claim that = . Two samples are randomly selected...

Calculate the test statistic F to test the claim that = . Two samples are randomly selected from populations that are normal. The sample statistics are given below. n1 = 25 n2 = 30 = 7.942 = 4.95

Let x be the number of courses for which a randomly selected student at a certain...

Let x be the number of courses for which a randomly selected student at a certain university is registered. The probability distribution of x appears in the table shown below: x 1 2 3 4 5 6 7 p(x) .05 .03 .09 .26 .37 .16 .04 (a) What is P(x = 4)? P(x = 4) = (b) What is P(x 4)? P(x 4) = (c) What is the probability that the selected student is taking at most five courses? P(at...

The probability that a randomly selected voter will answer "Yes" to a certain political question is...

The probability that a randomly selected voter will answer "Yes" to a certain political question is 5/9. 720 voters are chosen at random, and each is asked the question. What is the probability that between 380 and 420 answer "Yes"? More than 360 answer "Yes"? Answer should be, respectively: .8664, .9987

The probability that a randomly selected box of a certain type of cereal has a particular...

The probability that a randomly selected box of a certain type of cereal has a particular prize is 0.2. Suppose you purchase box after box until you have obtained four of these prizes. (a) What is the probability that you purchase x boxes that do not have the desired prize? h(x; 4, 0.2) b(x; 4, 2, 10) nb(x; 4, 2, 10) b(x; 4, 0.2) h(x; 4, 2, 10) nb(x; 4, 0.2) (b) What is the probability that you purchase...

Question