JEE Advanced Mathematics question
The number of 6-digit numbers that can be made with the digits 1, 2, 3 and 4 and having exactly two pairs of digits is
Answer: (C)
Solution
Six places, digits from , exactly two digits occurring exactly twice. These two pairs fill 4 places; the remaining 2 places must be filled by the two other digits, once each (using one of them twice would create a third pair, and adding to a pair would destroy it). So the digit pattern is
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