Let's break this down. We're going to add 6791 and 80 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 9 & 8 in column 2 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 7 & 0 in column 3 and get 8.

Step 4: We'll start by adding the digits 6 & 0 in column 4 and get 6.

So, 6791 + 80 = 6871.

