Let's break this down. We're going to add 7511 and 5820 together.

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

Step 2: We'll start by adding the digits 1 & 2 in column 2 and get 3.

Step 3: We'll start by adding the digits 5 & 8 in column 3 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 7 & 5 in column 4 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

So, 7511 + 5820 = 13331.

