Mathematics
A book shelf manufacturer claims that there largest bookshelves can withstand an average of 245 pounds of books, with a standard deviation of about 30 pounds. A sample of 81 bookshelves is selected and the weight they could withstand was measured.

a) what is the probability that the value of the sample mean is less than 247 or greater than 252.4

b) what is the probability that the value of the sample mean is between 245.4 and 248.7?

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