IMO Shortlist 1998 problem N3
Dodao/la:
arhiva2. travnja 2012. Determine the smallest integer

for which one can choose four different numbers

and

from any

distinct integers such that

is divisible by

.
%V0
Determine the smallest integer $n\geq 4$ for which one can choose four different numbers $a,b,c$ and $d$ from any $n$ distinct integers such that $a+b-c-d$ is divisible by $20$.
Izvor: Međunarodna matematička olimpijada, shortlist 1998