Maths Olympiad Prep

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Combinatorics Difficulty 4.8 AIME Find the answer United States

Problem:

This question forms a three question multiple choice test. After each question, there are 4 choices, each preceded by a letter. Please write down your answer as the ordered triple (letter of the answer of Question #1\# 1, letter of the answer of Question #2\# 2, letter of the answer of Question \#3). If you find that all such ordered triples are logically impossible, then write "no answer" as your answer. If you find more than one possible set of answers, then provide all ordered triples as your answer.
When we refer to "the correct answer to Question XX " it is the actual answer, not the letter, to which we refer. When we refer to "the letter of the correct answer to question XX " it is the letter contained in parentheses that precedes the answer to which we refer.
You are given the following condition: No two correct answers to questions on the test may have the same letter.

Question 1. If a fourth question were added to this test, and if the letter of its correct answer were (C), then:
(A) This test would have no logically possible set of answers.
(B) This test would have one logically possible set of answers.
(C) This test would have more than one logically possible set of answers.
(D) This test would have more than one logically possible set of answers.

Question 2. If the answer to Question 2 were "Letter (D)" and if Question 1 were not on this multiple-choice test (still keeping Questions 2 and 3 on the test), then the letter of the answer to Question 3 would be:
(A) Letter (B)
(B) Letter (C)
(C) Letter (D)
(D) Letter (A)

Question 3. Let P1=1P_{1}=1. Let P2=3P_{2}=3. For all i>2i>2, define Pi=Pi1Pi2Pi2P_{i}=P_{i-1} P_{i-2}-P_{i-2}. Which is a factor of P2002P_{2002} ?
(A) 3
(B) 4
(C) 7
(D) 9

Multiple choice: answer with the letter of the option you want.

Solution

Solution:

(A, C, D).

Question 2: In order for the answer to be consistent with the condition, "If the answer to Question 2 were Letter (D)," the answer to this question actually must be "Letter (D)." The letter of this answer is (C).

Question 1: If a fourth question had an answer with letter (C), then at least two answers would have letter (C) (the answers to Questions 2 and 4). This is impossible. So, (A) must be the letter of the answer to Question 1.

Question 3: If we inspect the sequence PiP_{i} modulo 3,4,73,4,7, and 99 (the sequences quickly become periodic), we find that 3,73,7, and 99 are each factors of P2002P_{2002}. We know that letters (A) and (C) cannot be repeated, so the letter of this answer must be (D).

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.