An electrical firm manufactures light bulbs that have a length of life that is normally distributed with mean equal to 799 hours and a standard deviation of 49 hours. Find the probability that a bulb burns between 769 and 878 hours.
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We use Z calculations
Z(769)= (769 - 799) / 49 = -0.61224898
Z(878)=(878 - 799) / 49 = 1.612244898
P(769 < X < 878)=P (-0.61 < Z < 1.61)
From the normal table we obtain
(0.94630 - 0.27093) = 0.67537
Thus our answer is 0.67537