Let's break this down. We're going to add 1278 and 2120 together.

Step 1: We'll start by adding the digits 8 & 0 in column 1 and get 8.

Step 2: We'll start by adding the digits 7 & 2 in column 2 and get 9.

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

Step 4: We'll start by adding the digits 1 & 2 in column 4 and get 3.

So, 1278 + 2120 = 3398.

