1. The problem is to find the sum of 15 and 15. 2. The formula for addition is $a + b = c$, where $a$ and $b$ are the numbers to add, and $c$ is the result. 3. Substitute the values: $15 + 15$. 4. Calculate the sum: $15 + 15 = 30$. 5. Therefore, the answer is 30.