Let's break this down. We're going to add 8209 and 420 together.

Step 1: We'll start by adding the digits 9 & 0 in column 1 and get 9.

Step 2: We'll start by adding the digits 0 & 2 in column 2 and get 2.

Step 3: We'll start by adding the digits 2 & 4 in column 3 and get 6.

Step 4: We'll start by adding the digits 8 & 0 in column 4 and get 8.

So, 8209 + 420 = 8629.

