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Copy Machine Hours and Annual Textbook Sales

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The time, in hours, that a copying machine may work without breaking down is a normally distributed random variable with the mean 549 and the standard deviation 68. Find the probability that the machine will work for at least 500 hours without breaking down - explain your answer (study guide problem 59).

Total annual textbook sales in a certain discipline are normally distributed. 45% of the time, sales are above 671,000 copies, and 10% of the time the sale are above 712,000 copies. Find the mean and the variance of annual sales - explain your answer (study guide problem 72).

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