Q) How many 5- digit telephone numbers can be formed that have at least one of their digits repeated.

A)
(A phone number doesn't begin with 0.)

Number of phone numbers with at least one repeated digit
= (Total ways in which a phone number can be made) - (number of phone numbers with no repeated digits)

= 9*10*10*10*10 - 9*9*8*7*6
= 9.(10^4) - 9.(9P4)
= 62784

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