1. **State the problem:** Calculate the sum of 12 and 13. 2. **Formula used:** Addition of two numbers is given by $a + b$. 3. **Apply the formula:** Here, $a = 12$ and $b = 13$, so we calculate: $$12 + 13$$ 4. **Perform the addition:** $$12 + 13 = 25$$ 5. **Final answer:** The sum of 12 and 13 is 25.