2. Let $a_{i}(i=1,2,3,4)$ be real numbers. If the sum of the elements of all non-empty proper subsets of the set $\left\{a_{1}, a_{2}, a_{3}, a_{4}\right\}$ is
28, then $a_{1}+a_{2}+a_{3}+a_{4}=$ $\qquad$
Answer: 4
Explanation: The number of non-empty proper subsets containing the element $a_{i}(i=1,2,3,4)$ is 7, so the sum of the elements of all non-empty proper subsets of $\mathrm{I}=\left\{a_{1}, a_{2}, a_{3}, a_{4}\right\}$ is $7\left(a_{1}+a_{2}+a_{3}+a_{4}\right)=28$, thus $a_{1}+a_{2}+a_{3}+a_{4}=4$.