1. The problem is to find the sum of 15 and 18. 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 given numbers: $15 + 18$. 4. Add the numbers: $15 + 18 = 33$. 5. Therefore, the sum of 15 and 18 is $33$.