1. The problem is to find the sum of 900 and 800. 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: $900 + 800$. 4. Add the numbers: $900 + 800 = 1700$. 5. Therefore, the sum of 900 and 800 is $1700$.