An electrical firm manufactures light bulbs that have a length of life that is approximately normally distributed, with mean equal to 786 hours and a standard deviation of 80 hours. Find the probability that a random samples of 17 bulbs will have an average life between 799 and 802 hours.
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We use Z calculations
z(799) = (799 - 786)/80 = 0.1625
z(802) = ( 802 - 786)/80 = 0.2
We then define
P (0.1625 < z < 0.2), using the normal table we have
( 0.57926 - 0.56356) = 0.01561 which is the required probability.