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Two numbers are equivalent if they have the same digits, for example $2271$ and $1227$ are equivalent. How many equivalent classes are there between $1000$ & $9999$?

I'm not sure I fully understand how this works, is the answer to this question $\dfrac{9\times 10\times 10\times 10}{4!}$? First calculating the total numbers formed and divindg by $4!$ to get rid of the repetitions.

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    Hint: The number of repetitions for the digits 1227 is not 4!.2010-11-18

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