2024 AIME II Problem 2

Attempt Problem 2 of the 2024 AIME II 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 2024 AIME II solutions, or check the answer key.

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2.

A list of positive integers has the following properties:

• The sum of the items in the list is 30.30.

• The unique mode of the list is 9.9.

• The median of the list is a positive integer that does not appear in the list itself.

Find the sum of the squares of all the items in the list.

Answer: 236
Concepts:modemedian (data)casework
Difficulty rating: 2180
Solution:

The median is an integer that is not in the list, so the list cannot have odd length (then the median would be a member). The unique mode 99 appears at least twice. Two items 9,99, 9 sum to 18,18, not 30,30, so try four items a<b<9a \lt b \lt 9 together with 9,9,9, 9, where aa and bb are distinct (a repeat would tie the mode) and a+b=12.a + b = 12. The median b+92\frac{b + 9}{2} must be an integer, so bb is odd, and a=12b<ba = 12 - b \lt b forces b>6.b \gt 6. Thus b=7b = 7 and a=5:a = 5: the list 5,7,9,95, 7, 9, 9 has median 8,8, which indeed does not appear.

No longer list works. With exactly two 99s, six items would need four distinct other values summing to 12,12, namely {1,2,3,6}\{1, 2, 3, 6\} or {1,2,4,5},\{1, 2, 4, 5\}, but both give median 4.5;4.5; eight or more items are impossible because six distinct positive values already sum to at least 21>12.21 \gt 12. With three 99s the remaining items sum to 3,3, and every option either puts 99 at the median or ties the mode. Four 99s already sum to more than 30.30.

The sum of squares is 25+49+81+81=236.25 + 49 + 81 + 81 = 236.

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