1989 AMC 8 Problem 22

Attempt Problem 22 of the 1989 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 1989 AMC 8 solutions, or check the answer key.

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

22.

The letters A,A, J,J, H,H, S,S, M,M, EE and the digits 1,1, 9,9, 8,8, 99 are each cycled separately (shifted one place at a time) to build a numbered list. Starting from AJHSME 1989,1989, the list begins: line 11 is JHSMEA 9891,9891, line 22 is HSMEAJ 8919,8919, line 33 is SMEAJH 9198,9198, and so on. On what numbered line will AJHSME 19891989 appear for the first time?
66
1010
1212
1818
2424
Answer: C
Concepts:least common multiple
Small hint:
The six letters return to AJHSME every 66 lines; the four digits return to 19891989 every 44 lines
Big hint:
Both return together on the least common multiple of 66 and 44
Solution:
The six letters cycle back to their original order AJHSME every 66 lines, and the four digits cycle back to 19891989 every 44 lines.
Both happen on the same line at the least common multiple of 66 and 4,4, which is 12.12. So AJHSME 19891989 first reappears on line 12.12.
Thus, the correct answer is C .
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