Let's solve this addition problem. We have 403 and 800, and we need to add them together.

Step 1: We'll start by adding the digits 3 & 0 in column 1 and get 3.

Step 2: We'll start by adding the digits 0 & 0 in column 2 and get 0.

Step 3: We'll start by adding the digits 4 & 8 in column 3 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

So, 403 + 800 = 1203.

