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subsets of {1,2,3, ...,1441} with 77 elements with sum even

Source: 7th Gulf Math Olympiad 2019 GMO p3

December 31, 2019
combinatoricsSubsetsSumProduct

Problem Statement

Consider the set S={1,2,3,...,1441}S = \{1,2,3, ...,1441\}. 1. Nora counts thoses subsets of SS having exactly two elements, tbe sum of which is even. Rania counts those subsets of SS having exactly two elements, the sum of which is odd. Determine the numbers counted by Nora and Rania. 2. Let tt be the number of subsets of SS which have at least two elements and the product of the elements is even. Determine the greatest power of 22 which divides tt. 3. Ahmad counts the subsets of SS having 7777 elements such that in each subset the sum of the elements is even. Bushra counts the subsets of SS having 7777 elements such that in each subset the sum of the elements is odd. Whose number is bigger? Determine the difference between the numbers found by Ahmad and Bushra.