a) The distribution of the lifespan of electric lamps is approximately normal. Random variable X~N(2000 h, 250 h). What is the probability that a randomly selected light bulb will burn up? i) at least 1800 h ii) 1800-2200 hours     b) The company that manu

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a) The distribution of the lifespan of electric lamps is approximately normal. Random variable X~N(2000 h, 250 h).
What is the probability that a randomly selected light bulb will burn up?
i) at least 1800 h
ii) 1800-2200 hours

 

 

b) The company that manufactures car engine spark plugs has stated in its test that the durability of the plugs follows the normal distribution N(45000 km, 1600 km).


After how many driving kilometers do the plugs have to be replaced if we want to be 99.9% sure that the plugs will not broken before replacement?

 

Calculate with excell cells and justify your calculations.

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