1986 AIME Problems/Problem 12

Problem

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?

Solution

The maximum is , attained when . We must now prove that no such set has sum at least 62. Suppose such a set existed. Then can't have 4 or less elements, otherwise its sum would be at most .

But also, can't have at least 6 elements. To see why, note that of its subsets have at most 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.

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.