Okay, so I need to add 313.9 and 12.6 together. Let me start by writing down the numbers to make sure I line up the decimal points correctly. 

313.9 is the same as 313.90, right? Because adding a zero at the end doesn't change the value. So when I add 12.6 to that, I should align the decimals. Let me visualize it:

  313.90
+  12.60
--------
 

Wait, but 12.6 is the same as 12.60, so adding those two should be straightforward. Let me add the numbers column by column. Starting from the rightmost digit:

0 + 0 = 0 in the hundredths place.
9 + 6 = 15, so I write down 5 and carry over 1.
3 + 2 + 1 (carry) = 6 in the tenths place.
1 + 1 = 2 in the ones place.
3 + 0 = 3 in the tens place.
And the hundreds place remains 3.

So putting it all together, that gives me 326.50. But since the original numbers didn't have trailing zeros beyond the decimal, maybe I can drop the last zero. So 326.5. Let me check that again.

Alternatively, maybe I can do it step by step. 313.9 plus 10 is 323.9. Then add 2.6 more. 323.9 + 2.6 = 326.5. Yeah, that matches. So the answer should be 326.5. But wait, when I first added 313.90 + 12.60, I got 326.50. So both methods give the same result. Therefore, the correct answer is 326.5. I think that's it.
</think>