3. (8 points) Select 5 different numbers from 1 to 9, such that the sum of the selected 5 numbers is exactly half the sum of the 4 numbers that were not selected. The sum of the selected numbers is $\qquad$.


【Answer】Solution: $(1+2+3+4+5+6+7+8+9) \div(2+1)$
$$
\begin{array}{l}
=45 \div 3 \\
=15
\end{array}
$$

Answer: The sum of the selected numbers is 15.
Therefore, the answer is: 15.