SOLUTION: A light bulb manufacturer finds that 5% of his bulbs last more than 500 hours. An improvement in the process meant that the mean lifetime was increased by 50 hours . In a new test,

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Question 1191723: A light bulb manufacturer finds that 5% of his bulbs last more than 500 hours. An improvement in the process meant that the mean lifetime was increased by 50 hours . In a new test, 20% of bulbs now lasted longer than 500 hours.
Find the mean and standard deviation of the original process

Answer by ikleyn(52779) About Me  (Show Source):
You can put this solution on YOUR website!
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Looking at this problem, I'd say that its formulation is incorrect (or incomplete).

Would it be correct and complete, it must say that the standard deviation after improvement remained the same as before it.


Since the problem is silent about it (leaving it for the reader to guess), this problem is more like a puzzle than a Math problem.