Let's break this down. We're going to add 1604555116 and 403949004 together.

Step 1: We'll start by adding the digits 6 & 4 in column 1 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 1 & 0 in column 2 and get 2.

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

Step 4: We'll start by adding the digits 5 & 9 in column 4 and get 14.
We'll write down the last digit 4 and carry the 1 to the next column.

Step 5: We'll start by adding the digits 5 & 4 in column 5 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 6: We'll start by adding the digits 5 & 9 in column 6 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

Step 7: We'll start by adding the digits 4 & 3 in column 7 and get 8.

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

Step 9: We'll start by adding the digits 6 & 4 in column 9 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 10: We'll start by adding the digits 1 & 0 in column 10 and get 2.

So, 1604555116 + 403949004 = 2008504120.

