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How many five-digit numbers beginning in a 6 have exactly one pair of identical digits?

 May 12, 2022
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How many five-digit numbers beginning in a 6 have exactly one pair of identical digits?

 

6 ** #$

 

10 posibilities for the double, 4C2 places they can go

9 possibilities for the third number and   8 possibilities for the fourth number

 

10 * 4C2 * 9 * 8 = 4320 such numbers

 May 12, 2022

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