Employees of a certain company are each to receive a unique 6-digit identification code consisting of the digits 0, 1, 2, 3, 4, and 5 such that no digit is used more than once in any given code. In valid codes, the second digit in the code is exactly twice the first digit. How many valid codes are there?
Choose the correct choice from the following choices: A) 48 B) 56 C) 64 D) 72 E) 80
There are 4! ways to make codes starting with 12.   There are 4! ways to make codes starting with 24.   The number of codes is 2*4! = 48.   The answer is 48.
<result>A</result>
Employees of a certain company are each to receive a unique 6-digit identification code consisting of the digits 0, 1, 2, 3, 4, and 5 such that no digit is used more than once in any given code. In valid codes, the second digit in the code is exactly twice the first digit. How many valid codes are there?