Let's break this down. We're going to add 6591 and 5399 together.

Step 1: We'll start by adding the digits 1 & 9 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 9 & 9 in column 2 and get 19.
We'll write down the last digit 9 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 5 & 3 in column 3 and get 9.

Step 4: We'll start by adding the digits 6 & 5 in column 4 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

So, 6591 + 5399 = 11990.

