2023 AIME I Problem 14

Below is the professionally curated solution for Problem 14 of the 2023 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2023 AIME I solutions, or check the answer key.

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Concepts:graph theorymodular arithmeticcombinations

Difficulty rating: 3500

14.

The following analog clock has two hands that can move independently of each other.

Initially, both hands point to the number 12.12. The clock performs a sequence of hand movements so that on each movement, one of the two hands moves clockwise to the next number on the clock face while the other hand does not move.

Let NN be the number of sequences of 144144 hand movements such that during the sequence, every possible positioning of the hands appears exactly once, and at the end of the 144144 movements, the hands have returned to their initial position. Find the remainder when NN is divided by 1000.1000.

Solution:

Record the hands as an ordered pair (a,b)Z12×Z12;(a, b) \in \mathbb{Z}_{12} \times \mathbb{Z}_{12}; each movement replaces (a,b)(a, b) by (a+1,b)(a+1, b) or (a,b+1),(a, b+1), so a valid sequence is a closed tour through all 144144 positions — equivalently, a choice, at each position, of which hand moves next. Sort the positions into 1212 rows according to b,b, and let SbZ12S_b \subseteq \mathbb{Z}_{12} be the set of aa-values at which the tour leaves row bb (a bb-move). A bb-move from row b1b - 1 enters row bb at the same aa-value, and the tour then runs through consecutive aa-values until its next exit. For these runs to cover row bb exactly once they must partition Z12,\mathbb{Z}_{12}, which forces each entry point to sit one step past an exit: Sb1=Sb+1.S_{b-1} = S_b + 1. In particular every SbS_b has the same size c,c, and Sb=S0b.S_b = S_0 - b.

Leaving row bb at its ii-th exit (in cyclic order) leads to a run ending at the (i+1)(i+1)-st exit of row b+1:b + 1: each bb-move advances the row index by 11 modulo 1212 and the exit index by 11 modulo c.c. The tour therefore closes after lcm(12,c)\operatorname{lcm}(12, c) bb-moves, while a full tour must use all 12c12c exits, so the tour is a single cycle through all 144144 positions precisely when gcd(c,12)=1.\gcd(c, 12) = 1. Conversely, every choice of S0S_0 with gcd(S0,12)=1\gcd(|S_0|, 12) = 1 yields exactly one valid movement sequence from the starting position.

Hence N=(121)+(125)+(127)+(1211)=12+792+792+12=1608,N = \binom{12}{1} + \binom{12}{5} + \binom{12}{7} + \binom{12}{11} = 12 + 792 + 792 + 12 = 1608, and the remainder when NN is divided by 10001000 is 608.608.

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