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Question

The number of four digit numbers that can be formed from the digit 0,1,2,3,4,5 with at least one digit repeated is ?

A
420
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B
560
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C
750
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D
None
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Solution

The correct option is D 750

Since we are looking for 44-digit numbers, the first (thousands) digit can't be 00.

Let's split cases:

Case 1: first digit is 11.In this case, we pick 33 digits in order out of the remaining 77, there are 765=210765=210 possibilities.

Case 2: first digit is not 11.

There are 66 ways to choose the first digit (2,3,...,72,3,...,7), 33 ways to place the digit 11 (hundreds, tens, ones), and 6565 (pick 22 in order from the remaining 66 digits) ways to choose the other digits. So there are 6365=5406365=540 possibilities in this case.

Total In total, there are 210+540=750210+540=750 possibilities to choose the number.


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