Radio stations in a certain country use a sequence of 5 or 6 letters as their station identification call letters. the first letter must be upper y comma upper n comma upper a comma upper q comma or upper r. assume there are no restrictions on the remaining letters, and repetition is allowed. a) how many 5-letter station identifications are possible? b) how many 6-letter station identifications are possible? c) how many total station identifications are possible? d) the identification for a randomly-chosen radio station is 5 letters in length. what is the probability that all five letters are different?