A 4-letter code word consists of letters A, B. If the code includes all the three letters, how many such codes are possible?
Choose the correct answer:
A) 72 B) 12 C) 36 D) 24 E) 18
Note that the correct answer to this question is 36, not 72.   AB-AB can be arranged in 4!/2!=12 ways;;   Total: 12.   Answer: 12.
<result>B</result>
A 4-letter code word consists of letters A, B. If the code includes all the three letters, how many such codes are possible?