Age lightbulb is supposed to last for 1,200 hours. in fact, light bulbs of this type last only 1,185 hours with a standard deviation of 70 hours. what is the probability that a sample of 100 lightbulbs will have an average life of at least 1,200 hours? mathematics

This can be solved using the formula for t test which is = x - mu over standard deviation divided by the square root of the number of samples. In this case, it is 1185 - 1200 over 70 divided by square root of 100 which give us a value of 2.14 which is equal to .9839 in area. Only rarely, just over one time in a hundred tries of 100 light bulbs, would the average life exceed 1200 hours.