The results stated in the previous problem generalize. If Ti, ..., T, are independent erponen- tial(X;) distributed random variables, then min {Ti, ..., to} ~ exponential(X1 + ... + An), and P(Ti = min (T1, . .., In)) = 14." . Assuming these, solve the following problem. A spacecraft has three engines. It can keep travelling if at least two of the engines are working. Suppose that the failure times of the engines are exponential with means 1 year, 1.5 years, and 3 years. What is the average length of time the spacecraft can travel