Hi Vincen,

We're told that all of the telephone extensions in a certain company must be EVEN numbers, and each of the extensions uses each of the four of the digits 1, 2, 3, and 6. We're asked for the number of four-digit extensions that the company can have under these conditions. This question can be solved with a Permutation or by simply listing out all of the possibilities:

Since we're asked for all of the 4-digit numbers, each of the digits (1, 2, 3 and 6) must be used exactly ONCE. Since the 4-digit number must be EVEN, the 4th digit must be either 2 or 6. Starting with that 'restriction', there are 2 options for the 4th digit. Once we place one of those numbers....
There will then be 3 remaining options for the 1st digit. Once we place one...
There will then be 2 remaining options for the 2nd digit. Once we place one...
There will then be 1 remaining option for the 3rd digit.

Total options = (2)(3)(2)(1) = 12 possible 4-digit numbers.

Final Answer: C

GMAT assassins aren't born, they're made,
Rich