15. Two devices, the lifetime of which are independent exponential random variables with parameters and ,

15. Two devices, the lifetime of which are independent exponential random variables with parameters λ and μ, are put on a life test. Suppose that the time until a catastrophe, such as a shock, that will fail both devices independently of their lifetimes is exponential with mean 1/γ. Find the probability that both devices will fail at the same time.