Question# 3: Two machines, labeled X and Y, have exponentially distributed lifetimes. If the two lifetimes of the two machines are independent, and the mean lifetime of bulb X is 600 hours, while the mean lifetime of bulb Y is 200 hours. What is the mean time to a machine failure Question# 4: Prove that the interarrival times of the Poisson process are exponentially distributed with mean λ