question

hello everyone! i have a question for problem 1510D.Digits: https://mirror.codeforces.com/contest/1510/problem/D. I saw the tutorial for this problem and i don't understand the sentence: "It’s easy to see it’s never optimal to remove more than 3 numbers since there are only 4 different remainders modulo 5." I have drafted for many times but i don't understand why it's never optimal to chose more than 3 numbers. Thank for your answers!