OK, let's do this. We've got 6555 and 9613 and we're adding them all together.

Step 1: We'll start by adding the digits 5 & 3 in column 1 and get 8.

Step 2: We'll start by adding the digits 5 & 1 in column 2 and get 6.

Step 3: We'll start by adding the digits 5 & 6 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 6 & 9 in column 4 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

So, 6555 + 9613 = 16168.

