2000 AIME I Problem 15

Below is the professionally curated solution for Problem 15 of the 2000 AIME I, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2000 AIME I solutions, or check the answer key.

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Concepts:permutationsprocess simulationpattern recognition

Difficulty rating: 3060

15.

A stack of 20002000 cards is labelled with the integers from 11 to 2000,2000, with different integers on different cards. The cards in the stack are not in numerical order. The top card is removed from the stack and placed on the table, and the next card is moved to the bottom of the stack. The new top card is removed from the stack and placed on the table, to the right of the card already there, and the next card in the stack is moved to the bottom of the stack. The process — placing the top card to the right of the cards already on the table and moving the next card in the stack to the bottom of the stack — is repeated until all cards are on the table. It is found that, reading from left to right, the labels on the cards are now in ascending order: 1,2,3,,1999,2000.1, 2, 3, \ldots, 1999, 2000. In the original stack of cards, how many cards were above the card labelled 1999?1999?

Solution:

Number the original positions 11 (top) through 20002000 (bottom) and put them in a queue. Each step removes the front position (which receives the next label 1,2,3,1, 2, 3, \ldots) and sends the new front to the back. So the card labelled 19991999 is the next-to-last card removed, and we must find which original position survives that long.

The first pass removes the odd positions 1,3,,19991, 3, \ldots, 1999 (labels 11 through 10001000) and, since it ends by sending 20002000 to the back, the next pass again starts by removing the front of the queue 2,4,,2000.2, 4, \ldots, 2000. Successive passes therefore remove 2,6,,19982, 6, \ldots, 1998 (the positions 2mod4\equiv 2 \bmod 4), then 4,12,,1996,4, 12, \ldots, 1996, then 8,24,,1992,8, 24, \ldots, 1992, then 16,48,,1968,200016, 48, \ldots, 1968, 2000 (the 6363 positions 16mod32\equiv 16 \bmod 32). That last pass ran through an odd number (125125) of cards, so the alternation shifts: the 6262 surviving multiples of 3232 now sit in the queue as 64,96,,1984,32.64, 96, \ldots, 1984, 32.

Continuing the same removal pattern from that queue, the next rounds remove 64,128,,1984;64, 128, \ldots, 1984; then 96,224,,1888,32;96, 224, \ldots, 1888, 32; then 288,544,,1824;288, 544, \ldots, 1824; then 160,672,1184,1696;160, 672, 1184, 1696; then 416,1440;416, 1440; and the final two cards removed are 928928 and 1952.1952. So label 19991999 goes to the card at original position 928,928, which had 927927 cards above it.

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