1. The problem statement, all variables and given/known data A manufacturing plant utilizes 3000 electric light bulbs whose length of life is normally distributed with mean 500 hours and standard deviation 50 hours. To minimize the number of bulbs that burn out during operating hours, all the bulbs are replaced after a given period of operation. How often should the bulbs be replaced if we want not more than 1% of the bulbs to burn out between replacement periods? 2. Relevant equations 3. The attempt at a solution I'm having great difficulty with this question. I'm confused as to how I can find how often the bulbs should be replaced (within what times interval?) I was wondering if this maybe is a combination of a normal distribution with an expontial ? I was thinking of finding standizing the distribution P(Z <= x-500/50)= 0.01 and then simply solving for x. But I am not sure how that even helps me. Overall I'm just quite confused with this problem. Thank you !