36 heat lamps are connected in a greenhouse so that when one lamp fails, another takes over immediately. (Only one lamp is turned on at any time.) The lamps operate independently, and each has a mean life of 50 hours and standard deviation of 4 hours. If the greenhouse is not checked for 1836 hours after the lamp system is turned on, what is the probability that the lamp system is still working (one lamp is burning) at the end of the 1836-hour period