Track / Stage 4 / 163 of 340 #423 of 1964

Problem 423

AMC 12 late, AIME early

Number theory Difficulty 4.8 Find the answer

(3) Given $\varnothing \varsubsetneqq M \subseteq \mathbf{N}$ , and if $x \in M$ , then $6-x \in M$ , then the set $M$ can contain at most
$\qquad$ elements

Your answer

A number or a short expression. Spacing, $ signs and \frac vs / are all fine.

Official solution

(3) 7

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.