Math

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Each digit of a four-digit PIN can be any number from 0 to 9. What is the percentage of PINs with no digit repeated more than twice?

  • Math -

    In a four digit number, a digit repeated twice means 3 of the numbers are the same.
    Repeated more than twice means repeated 3 times, or all four digits are the same.

    PINs with no digit repeated more than twice would therefore mean pins which do not have identical digits.
    Out of 10000 pins, there are only 10 PINs with identical digits, so the probability is
    9990/10000.
    Reduce the fraction for the proper answer.

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