Certain type of battery is known to last avg. of 1110 days w/ standard deviation of 80 days. Suppose that 400 batteries are selected. Find probability that the avg. length of life of the 400 batteries is between 1105 & 1110.

P ( 1105 < xbar < 1110 | mu = 1110, std error = 80/sqrt(400))

P ( 1105 < xbar < 1110 | mu = 1110, std error = 4)

=0.5 -0.1057 = 0.3943

Answer: 0.3943

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