The number of natural numbers smaller than 104 of which all digits are different, is

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The number of natural numbers smaller than 10⁴ of which all digits are different, is

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answer is 5274.

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Number of one digit different numbers=9

Number of two digit different numbers= $9.9 p_{1}$ =81

Number of three digit different numbers= $9.9 p_{2}$ =648

Number of four digit different numbers= $9.9 p_{3}$ =4536

$Number of natural numbers s = 9 + 9.9 p_{1} + 9.9 p_{2} + 9.9 p_{3}$

=9+81+648+4536=5274

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