October_7th_Practice_Problems

# October_7th_Practice_Problems - scores to be normally...

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October 7th Practice Problems 1. The downtime per day for the universitys computing server averages about 40 minutes and for the email server averages about 20 minutes. The downtimes were recorded for ﬁve randomly selected days, which showed standard deviations of 8 and 10 min- utes, respectively. The director of computer services is interested in estimating the probability of the sample average for computing server is larger by at least 15 minutes than that for email server. The director uses t-distribution with mean 20 and pooled standard deviation to compute the probability of interest. Although the assumption of equal population variances is reasonable, which condition is violated here, making this probability calculation invalid? 2. For an aptitude test for quality-control technicians in an electronics ﬁrm, history shows
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Unformatted text preview: scores to be normally distributed with a variance of 225. If 20 applicants are to take the test, ﬁnd an interval in which the sample variance of test scores should lie with probability 0.90. 3. Waiting times at a service counter in a pharmacy are exponentially distributed with a mean of 10 minutes. If 100 customers come to the service counter in a day, approximate the probability that at least half of them must wait for more than 10 minutes. 4. One-hour carbon monoxide concentrations in air samples from a large city average 12 ppm. The standard deviation of carbon monoxide concentrations measured from the last ﬁve air samples was 9ppm. Find the probability that the average concentration in next ﬁve samples selected randomly will exceed 14 ppm....
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