Find the number of all 6-digit numbers such that all the digits of each number are selected from the set {1, 2, 3, 4} and any digit that appears in the number appears at least twice
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- Total 6 digit numbers = 9X10X10X10X10X10 =9000000
- Possible numbers when no repetition is allowed is
- 9X9X8X7X6X5= 136080
- So required number is 763920 by subtracting the above two numbers
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