24. Let $k \geqslant 3$. Find all sets of positive integers $\left\{a_{1}, \cdots, a_{k}\right\}$ such that
(i) $a_{1}, \cdots, a_{k}$ are distinct positive integers;
(ii) the sum of any three of these numbers is divisible by each of the three numbers.
24. Let $1 \leqslant a_{1}<a_{2}<a_{3}$. If $a_{3}\left|a_{1}+a_{2}, a_{2}\right| a_{3}+a_{1}, a_{1} \mid a_{2}+a_{3}$, then $a_{3}=a_{1}+a_{2}, a_{2}=2 a_{1}$. All sets are $k=3,\left\{a_{1}, 2 a_{1}, 3 a_{1}\right\}$.