Let's break this down. We're going to add 5803 and 12 together.

Step 1: We'll start by adding the digits 3 & 2 in column 1 and get 5.

Step 2: We'll start by adding the digits 0 & 1 in column 2 and get 1.

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

Step 4: We'll start by adding the digits 5 & 0 in column 4 and get 5.

So, 5803 + 12 = 5815.

