Chapter 6: Problem 55
Find the following probabilities for the standard normal random variable:
a. \(P(.3
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Chapter 6: Problem 55
Find the following probabilities for the standard normal random variable:
a. \(P(.3
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a. Find a \(z_{0}\) such that \(P\left(-z_{0} \leq z \leq z_{0}\right)=.95\). b. Find a \(z_{0}\) such that \(P\left(-z_{0} \leq z \leq z_{0}\right)=.98\).
It is known that \(30 \%\) of all calls coming into a telephone exchange are long-distance calls. If 200 calls come into the exchange, what is the probability that at least 50 will be long-distance calls?
The life span of oil-drilling bits depends on the types of rock and soil that the drill encounters, but it is estimated that the mean length of life is 75 hours. Suppose an oil exploration company purchases drill bits that have a life span that is approximately normally distributed with a mean equal to 75 hours and a standard deviation equal to 12 hours. a. What proportion of the company's drill bits will fail before 60 hours of use? b. What proportion will last at least 60 hours? c. What proportion will have to be replaced after more than 90 hours of use?
A researcher notes that senior corporation executives are not very accurate forecasters of their own annual earnings. He states that his studies of a large number of company executive forecasts "showed that the average estimate missed the mark by \(15 \%\)." a. Suppose the distribution of these forecast errors has a mean of \(15 \%\) and a standard deviation of \(10 \%\). Is it likely that the distribution of forecast errors is approximately normal? b. Suppose the probability is .5 that a corporate executive's forecast error exceeds \(15 \% .\) If you were to sample the forecasts of 100 corporate executives, what is the probability that more than 60 would be in error by more than \(15 \% ?\)
Find the normal approximation to \(P(355 \leq x \leq 360)\) for a binomial probability distribution with \(n=400\) and \(p=.9\).
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