4 thoughts on “PnC Examples 1

    • If all are identical then there would be only one way of arranging since any arrangement would look the same. For identical objects, we are more concerned with dividing them into distinct groups, a topic touched upon in a later post.


  1. Sir, I remember reading this post and went with the same reasoning for this Question in mock: 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?

    The total no of ways we get are 4860, and I typed in 4859 using the same logic we did for lock Question. Won’t he take 4859 trials to be certain since the last one would obviously be correct, Sir?

    • No. He still has to dial the number to succeed. If the above question had been “how many trials are needed to open the lock?” then the answer would have been 625.

      If the question had been “…to come to know the correct number?” then I would agree with your logic.


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