An engineering professor acquires a new computer once every 2 years. The professor can choose from three models: M1, M1, and M3. If the present model is M1, the next model can be M2 with probability 0.2, or M3 with probability 0.15. If the present model is M2, the probabilities of switching to M1 and M3 are 0.6 and 0.25, respectively. And, if the present model is M3, then the probabilities of purchasing M1 and M2 are 0.5 and 0.1, respectively. Represent the situation as a Markov chain.