MathNet 2026 — Part 07 Question 40

Let $M$ be a subset of the set of 2021 integers $\{1,2,3, \ldots, 2021\}$ such that for any three elements (not necessarily distinct) $a, b, c$ of $M$ we have $|a+b-c|\gt 10$. Determine the largest possible number of elements of $M$.