A submarine has three navigational devices but can remain at sea if at least two are working. Suppose that the failure times are exponential with means 1, 1.5, and 3 years. Formulate a Markov chain with states 0 D all parts working, 1,2,3 D one part failed, and 4D two failures. Compute E0T4 to determine the average length of time the boat can remain at sea.