Let's break this down. We're going to add 125 and 3331 together.

Step 1: We'll start by adding the digits 5 & 1 in column 1 and get 6.

Step 2: We'll start by adding the digits 2 & 3 in column 2 and get 5.

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

Step 4: We'll start by adding the digits 0 & 3 in column 4 and get 3.

So, 125 + 3331 = 3456.

