After one bulb has failed,_ bulbs are still burning. Furthermore, regardless of the time at which the first bulb failed, it follows from the memoryless property of the exponential distribution that the distribution of the remaining lifetime of each of the other _ bulbs is still an exponential distribution with parameter _ . In other words, the situation that is the same as it would be if we were starting the test over again from time t=0 with _ new bulbs