In: Statistics and Probability
This exercise uses the normal probability density function and
requires the use of either technology or a table of values of the
standard normal distribution.
The cash operating expenses of the regional phone companies during
the first half of 1994 were distributed about a mean of $29.88 per
access line per month, with a standard deviation of $2.65. Company
A's operating expenses were $27.00 per access line per month.
Assuming a normal distribution of operating expenses, estimate the
percentage of regional phone companies whose operating expenses
were closer to the mean than the operating expenses of Company A
were to the mean. (Round your answer to two decimal places.)
The difference between the mean operating expenses of all regional phone companies and mean operating expenses of Company A = $29.88 - $27.00 = $2.88
Let X be the random variable denoting the operatind expense of regional phone companies.
X ~ N(29.88, 2.65) i.e. Z = (X - 29.88)/2.65 ~ N(0,1)
The probability that regional phone companies whose operating expenses were closer to the mean than the operating expenses of Company A were to the mean = P[|X - 29.88| 2.88] = P[|X - 29.88|/2.65 2.88/2.65] = P[|Z| 1.0868] = P[-1.0868 Z 1.0868] = (1.0868) - (-1.0868) = 0.8614 - 0.1386 = 0.7228
[(.) is the cdf of N(0,1)]
Hence, the required percentage is 72.28%.
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