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Math Help - Easy probability question

  1. #1
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    Easy probability question

    Postal codes have the form LDL DLD (where L is any letter from A-Z and D is any digit from 0-9). Suppose a computer program generates postal codes randomly in such a way that no two generated postal codes are dependent on each other. What is the probability that the program will generate the same postal code two times in row?

    I figured that there were 17,576,000 different codes. So I thought the probability of getting the same one twice was 2/17,576,000, however the answer shows 1/17,576,000.

    Can anyone explain why the numerator is just 1? Thanks for the help!
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  2. #2
    Pim
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    Okay, so this program will generate two postal codes.
    After it generates the first one, the second one has to be the exact same one. (Regardless of what the first one was.)
    There is only one postal code that is exactly the same, hence the probability is 1/17,576,000

    2/17,576,000 would be the chance that the machine generates one of 2 pre-specified postal codes.

    Hope this helps.
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