How many possible phone numbers does Danny have?
Danny forgot the seven-digit telephone number of his friend but he remembers the following information: the first three digits from the left are either 242 or 472, the digit 7 appears once and the number is an even number. If Danny were to use a trial and error method to reach his friend, what is the minimum number of trials that he has to make so that he can be certain of succeeding?
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