1986 AIME Problems/Problem 12
Let the sum of a set of numbers be the sum of its elements. Let be a set of positive integers, none greater than 15. Suppose no two disjoint subsets of have the same sum. What is the largest sum a set with these properties can have?
The maximum is , attained when . We must now prove that no such set has sum at least 62. Suppose such a set existed. Then must have more than 4 elements, otherwise its sum would be at most if it had 4 elements.
But also, can't have at least 6 elements. To see why, note that of its subsets have at most four elements (the number of subsets with no elements plus the number of subsets with one element plus the number of subsets with two elements plus the number of subsets with three elements plus the number of subsets with four elements), so each of them have sum at most 54. By the Pigeonhole Principle, two of these subsets would have the same sum, a contradiction to the givens.
Thus, must have exactly 5 elements. contains both 15 and 14 (otherwise its sum is at most ). It follows that cannot contain both and for any . So now must contain 13 (otherwise its sum is at most ), and cannot contain 12.
Now the only way could have sum at least would be if . But so this set does not work, a contradiction. Therefore 61 is indeed the maximum.
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