The first question uses the full probability formula, and the second question uses the Bayesian formula. (1) Let the incoming characters AAAA, BBBB and CCCC be events A, B and C respectively; The problem (1) is event D. Then p (a) =1/3p (b) =1/3p (c) =1/3p (d | a) = 0.6 * 0.2 * 0.6 = 0. Therefore, from the full probability formula, p (d) = p (ad)+p (BD)+p (CD) = p (a) p (d | a)+p (b) p (d | b)+p (c) p (d | c) = 0.008 (hint