Consider the five symbols A, B, C, D, E. A string of letters is written using these five in the following way. The letter A is written first. No letter is written twice in a row. After a letter is written, the next letter written is equally likely to be any of the other four. Formulate a difference equation to determine the probability p_n that the letter B is the nth letter written. Use the law of total probability. Specify the initial conditions