Let $A$ be a set of $65$ integers with pairwise different remainders modulo $2016$. Prove that exists a subset $B=\{a,b,c,d\}$ of set $A$ such that $a+b-c-d$ is divisible with $2016$
Problem
Source: Regional Olympiad - Federation of Bosnia and Herzegovina 2016
Tags: Combinatorial Number Theory, remainder, set, combinatorics