Let's solve this addition problem. We have 9904 and 1287, and we need to add them together.

Step 1: We'll start by adding the digits 4 & 7 in column 1 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 0 & 8 in column 2 and get 9.

Step 3: We'll start by adding the digits 9 & 2 in column 3 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 9 & 1 in column 4 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

So, 9904 + 1287 = 11191.

