Let's break this down. We're going to add 15 and 9021 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 1 & 2 in column 2 and get 3.

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

Step 4: We'll start by adding the digits 0 & 9 in column 4 and get 9.

So, 15 + 9021 = 9036.

