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Q)

The total number of 4 digit numbers in which the digits are in descending order is ?

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A)
4 digits are selected from any of the 10 numbers $0,1,2,.....9$.
Since the numbers are arranged in descending order $0$
does not come in first position.
No. of selection of any 4 numbers from $0,1,2,....9 =^{10}C_4$
Arrangement of the digits are in descending order which is only one way.
$\therefore$ The required no. of 4 digit numbers = $^{10}C_4$
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