1992 AMC 8 Problem 15

Attempt Problem 15 of the 1992 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1992 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

15.

What is the 19921992nd letter in this sequence?
ABCDEDCBAABCDEDCBAABCDEDCBAABCDEDC⋯\cdots
AA
BB
CC
DD
EE
Answer: C
Concepts:modular arithmeticpattern recognition
Small hint:
The string ABCDEDCBAABCDEDCBA repeats over and over; count how many letters are in one block
Big hint:
One block has 99 letters, so divide 19921992 by 99 and use the remainder to find the position within a block
Solution:
The sequence is the 99-letter block ABCDEDCBAABCDEDCBA repeated again and again.
Since 1992=9×221+3,1992 = 9 \times 221 + 3, the 19921992nd letter is the 33rd letter of a block, which is C.C.
Thus, the correct answer is C .
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