12. Given an arithmetic sequence $\left\{a_{n}\right\}$, the sum of the first 15 terms $S_{15}=30$. Then $a_{1}+a_{8}+a_{15}=$ $\qquad$
12.6.

From $S_{15}=30 \Rightarrow a_{1}+7 d=2$.
Therefore, $a_{1}+a_{8}+a_{15}=3\left(a_{1}+7 d\right)=6$.