2009 AIME II Problem 4

Below is the professionally curated solution for Problem 4 of the 2009 AIME II, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2009 AIME II solutions, or check the answer key.

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Concepts:arithmetic sequencefactorcasework

Difficulty rating: 2110

4.

A group of children held a grape-eating contest. When the contest was over, the winner had eaten nn grapes, and the child in kkth place had eaten n+22kn + 2 - 2k grapes. The total number of grapes eaten in the contest was 2009.2009. Find the smallest possible value of n.n.

Solution:

Let cc be the number of children. The grape counts n,n, n2,n - 2, ,\ldots, n+22cn + 2 - 2c form an arithmetic sequence, so the total is cc times the average of the first and last terms: cn+(n+22c)2=c(n+1c)=2009=7241.c \cdot \frac{n + (n + 2 - 2c)}{2} = c(n + 1 - c) = 2009 = 7^2 \cdot 41. Thus c2009c \mid 2009 and n=2009c+c1.n = \frac{2009}{c} + c - 1.

The last-place child ate n+22c=2009c+1c0n + 2 - 2c = \frac{2009}{c} + 1 - c \ge 0 grapes, which forces c(c1)2009,c(c - 1) \le 2009, ruling out c=49,c = 49, 287,287, and 2009.2009. The remaining divisors give n=2009n = 2009 for c=1,c = 1, n=287+6=293n = 287 + 6 = 293 for c=7,c = 7, and n=49+40=89n = 49 + 40 = 89 for c=41.c = 41.

The smallest possible value is n=89.n = 89.

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