fails. So, let's find the probability of the opposite event and subtract that value from 1. The opposite event is when none of the 10 light bulbs fails, since the probability of each light bulb not to fail is 1-0.06=0.94 the the probability that none of the 10 light bulbs fails is 0.94^10. Hence, the probability that at least one light bulb fails is 1-0.94^10. Answer: E. Now, you should have spotted that your reasoning was not right because of one simple thing, consider the case when we have 100 light bulbs instead of 10, then according to your logic the probability that the string of light bulbs will fail would be 100*0.06=6, which is not possible since the probability of an event can not be more than 1 (100%). A string of 10 lightbulbs is wired in such a way that if any [ #permalink ] Wed May 16, 2012 2:41 am A string of 10 lightbulbs is wired in such a way that if any individal lightbulb fails, the entire string fails. If for each individual lightbulb the probability of failing during the period T is 0.06, what is the probability that the string of lightbulbs will fail during the period T? A) 0.06 B) (0.06)^10 C) 1 - (0.06)^10 D) (0.94)^10 E) 1 - (0.94)^10 I still believe it is B. Why exactly is it E? I understand that E is 1- the probability of NOT failing. But I would assume the B is just the probability