Bunuel
12 Days of Christmas 2024 - 2025 Competition with $40,000 of PrizesA certain five-letter code may only be constructed with A’s, B’s, and C’s. If an A appears in any position but the first, it must be immediately preceded by a B. If a B appears in any position but the first, it must be immediately preceded by a C. What is the fifth letter in the code?
(1) The first and fourth letters in the code are the same, as are the second and fifth letters.
(2) There is exactly one C in the five-letter code.
Statement (1):
The first and fourth letters in the code are the same, as are the second and fifth letters.Let the code be X1 X2 X3 X4 X5. From this statement:
This imposes some symmetry on the code, but we cannot deduce X1, X2, or X3 uniquely without further information. For example:
- X1=A, X2=C, and X3=B are valid under the rules, leading to ACBAC, with X5=C.
- X1=B, X2=A, and X3=C could lead to BACBA, with X5=A
Without further constraints, X5 cannot be determined uniquely.
Statement (1) alone is insufficient.Statement (2):[b]There is exactly one C in the five-letter code.[/b]
If there is only one C, it must be positioned in a way that allows the rules to be satisfied:
- A B must always follow a C if B is not in the first position.
- An A must always follow a B if A is not in the first position.
Let's try various positions for C
If we start with C, there is not possible combination, (CBA, the next two values post A can not be A or B according to the constraints, and we cannot add C due to the statement (2) condition)
If the second letter is C, there is not possible combination, (BCBA or ACBA, the next value post A can not be A or B according to the constraints, and we cannot add C due to the statement (2) condition)
If the third letter is C, there is not possible combination, (BACBA), therefore the 5th letter has to be A
Similarly, If the fourth letter or fifth letter is C, there is not possible combination
Therefore, only
Statement (2) is sufficient.