For certain types of fluorescent lights, the number of hours a bulb will burn before requiring replacement

Question:

For certain types of fluorescent lights, the number of hours a bulb will burn before requiring replacement has a mean of 3000 hours and a standard deviation of 250 hours. Suppose that 5000 of such bulbs are installed in an office building. Use the Chebychev inequality to estimate the number that will require replacement between 2000 and 4000 hours, inclusive, from the time of installation.