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Unformatted text preview: are replaced, after how many hours should all the bulbs be replaced? Need x = ? such that P( X < x ) = 0.09. o Find z such that P( Z < z ) = 0.09. The area to the left is 0.09 = ( z ). z = 1.34 . t x = + z . x = 2,000 + 300 ( 1.34 ) = 1,598 hours . c) (4) What is the probability that the average lifetime of six randomly and independently selected bulbs is over 2,180 hours? Since the distribution we sample from is normal (Case 2), . n Z X =- . P ( X > 2,180 ) = -> 6 300 000 , 2 180 , 2 Z P = P ( Z > 1.47 ) = 1 ( 1.47 ) = 0.0708 ....
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