Bunuel wrote:
Employees of a certain company are each to receive a unique 7-digit identification code consisting of the digits 0, 1, 2, 3, 4, 5, and 6 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?
(A) 42
(B) 120
(C) 210
(D) 360
(E) 840
Since the second digit is twice the first, the first two digits of a valid code can only be 12, 24, or 36.
For each of these first two digits, the
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