Taking the set of natural numbers as the universal set,write down the complement of the following set:
$A = \{ x : x \text{ is a positive multiple of } 3 \}$

(N/A) Let $U = N$ be the set of natural numbers.
The complement of a set $A$,denoted by $A'$,is defined as $A' = \{ x : x \in U \text{ and } x \notin A \}$.
Given $A = \{ x : x \text{ is a positive multiple of } 3 \}$.
Therefore,the complement $A' = \{ x : x \in N \text{ and } x \text{ is not a multiple of } 3 \}$.