We've got two numbers: 8406 and 6600. Let's find their sum.

Step 1: We'll start by adding the digits 6 & 0 in column 1 and get 6.

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 & 6 in column 3 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 8 & 6 in column 4 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

So, 8406 + 6600 = 15006.

