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Q.
How many four digit numbers are there with distinct digits?
J & K CETJ & K CET 2013Permutations and Combinations
Solution:
$ \overline{\begin{align} & 9\,\,\,digits \\ & (1\,to\,\,9) \\ \end{align}} $ $ \overline{\begin{align} & 9\,\,\,digits \\ & (0\,to\,\,9) \\ \end{align}} $ $ \overline{\begin{align} & 8\,\,\,digits \\ & (0\,to\,\,9) \\ \end{align}} $ $ \overline{\begin{align} & 7\,\,\,digits \\ & (0\,to\,\,9) \\ \end{align}} $ Except those digit used in first place Except those digits used in Ist and IInd place Except those digits used in Ist, IInd and IIIrd place $ \therefore $ Required number of digits $ =9\times 9\times 8\times 7 $ $ =4536. $