How many 4 digit numbers can be formed by using the digits 1, 2, 3, 4, 5, 6, 7 if at least one digit is repeated

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1561

1578

1580

1621

answer is A.

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Detailed Solution

No of 4 digit no’s $= 7^{4} = 2401$
4 digit no’s repeation allowed $= 7 P 4 = 840$
The numbers that can be formed when at least one digit is repeated $= 2401 - 840 = 1561$

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