2003 AIME I Problem 3

Attempt Problem 3 of the 2003 AIME I 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 2003 AIME I solutions, or check the answer key.

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

3.

Let the set S={8,5,1,13,34,3,21,2}.\mathcal{S} = \{8, 5, 1, 13, 34, 3, 21, 2\}. Susan makes a list as follows: for each two-element subset of S,\mathcal{S}, she writes on her list the greater of the set’s two elements. Find the sum of the numbers on the list.
Answer: 484
Concepts:counting pairsdouble counting
Small hint:
Each element is written on the list once for every smaller element in the set
Big hint:
Sort the set as 1,2,3,5,8,13,21,341, 2, 3, 5, 8, 13, 21, 34 and multiply each element by the count of smaller elements
Solution:
An element xx is the greater element of a two-element subset exactly once for each smaller element of the set, so xx contributes to the sum once per element below it. Sorting the set as 1,2,3,5,8,13,21,34,1, 2, 3, 5, 8, 13, 21, 34, the sum of the list is 0(1)+1(2)+2(3)+3(5)+4(8)+5(13)+6(21)+7(34)=2+6+15+32+65+126+238=484. \begin{aligned} &0(1) + 1(2) + 2(3) + 3(5) \\ &\quad {}+ 4(8) + 5(13) + 6(21) \\ &\quad {}+ 7(34) \\ &= 2 + 6 + 15 + 32 \\ &\quad {}+ 65 + 126 + 238 = 484. \end{aligned}
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