JEE Advanced Mathematics question

The number of 4-digit numbers that can be made with the digits 1, 2, 3, 4 and 5 in which at least two digits are identical, is

Answer: (B)

Solution

Count the complement: (all 4-digit numbers with repetition allowed) − (those with all digits different).

- Repetition allowed: . - All digits different:

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Question ID VK-108163