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n ' Consider a communication system which transmits the two digits 0 and 1 through several stages. Let xn, n? 1 be the digit leaving the nth stage of a system and x, be the digit entering the first stage (leaving the Oth stage). At each stage there is a constant probability q that the digit which enters will be transmitted unchanged and probability p otherwise. Find out the probability that the digit entering the first stage is zero given that the digit leaving the mth stage is 0.