Our department seminar room is lit by 7 light bulbs, controlled by a single switch. A minimum of 3 lights are needed for the room to be usable. Suppose the lifetime of the brand of light bulbs titted is exponentially distributed with a mean of 400 hours. Assume a seminar lasts exactly 1 hour. and that the lights are switched on only during the seminar. What is the probability that alter 500 seminars, exactly 3 bulbs do not need to be replaced? You may need to use a calculator. Give your answer rounded to 4 decimal places