Question
The number of 6digits numbers that can be made with the digits 1, 2, 3 and 4 and having exactly two pairs of digits is :

480

540

1080

None of these
easy
Solution
1080
To form two pairs, select 2 digits from 4 digits in ^{4}C_{2 }ways and select 2 other digits in ^{2}C_{2} ways.
Then arrange them in ways
Number of number = ^{4}C_{2 }^{2}C_{2} × 6!/2! 2! = 1080
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