Okay, we are tasked with adding 599 and 5599. Let's begin!

Step 1: We'll start by adding the digits 9 & 9 in column 1 and get 18.
We'll write down the last digit 8 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 & 5 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 0 & 5 in column 4 and get 6.

So, 599 + 5599 = 6198.

