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There is a four digit code. Repetition of the same code is not allowed. How many possible combinations can be possible?
I tried it as follows,
As repetition of the same code is not allowed so it should be $10P4$ choices?

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    Can the starting digit be $0$?2011-10-24
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    @SrivatsanNarayanan: This is the same condition that I was thinking of. I found this question from a forum. I think we should neglect this condition.2011-10-24
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    The question is not that clear. Whenever you say "repetition of the same code", I presume you mean "repetition of the same digit". Do you know the answer to the question by any chance?2011-10-24
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    @SrivatsanNarayanan: Here repetition refers to repeating the same whole code again like 5555 can not come again in the series but 5554 can.2011-10-24
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    @Akito: Your last comment makes the question less clear, not more so. What is this "series"? It doesn't occur in the question. Is there more than one four-digit code? If so, how many? I've been reading several of your questions over the last couple of days, and there seems to be a general pattern of vague, unclear or sloppy formulations -- please put more care into formulating your questions; this is also in your own interest, as you will then get more useful answers.2011-10-24
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    @joriki: I am sorry. I will take good care of it from now own..2011-10-24
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    @FahadUddin Which digits are allowed to be included? Can it be all digits (0 through 9), or is it only some of these digits?2013-05-22

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