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subset of {1,2,..,100} with 48 elements has 11/(a+b) (HOMC 2009 J Q11)

Source:

July 25, 2019
combinatoricsnumber theorydivisible

Problem Statement

Let A={1,2,...,100}A = \{1,2,..., 100\} and BB is a subset of AA having 4848 elements. Show that BB has two distint elements xx and yy whose sum is divisible by 1111.