solution: We have \[
  \begin{array}{@{}c@{}c@{}c@{}c@{}c@{}c}
  & & & 1 & \\
  & 3 & 1 & 3. & 9 \\
  + & & 1 & 2. & 6
  \\ \cline{1-5}
  & 3 & 2 & 6. & 5 \\
  \end{array}
  \]Since $9+6=15$ in the right-most column is greater than 9, we record the 5 and "carry" the 10 to the next column as a 1 (shown over the second 3 in the addend 313.9). The answer is $\boxed{326.5}$.
prompt.content: <|im_start|>system
  You are a helpful assistant.<|im_end|>
  <|im_start|>user
  Add 313.9 to 12.6. Express the result as a decimal.
  Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
  <|im_start|>assistant
  
prompt.role: user
extra_info.answer: 326.5
extra_info.question: Add 313.9 to 12.6. Express the result as a decimal.
extra_info.split: test