Okay, we are tasked with adding 18 and 1503. Let's begin!

Step 1: We'll start by adding the digits 8 & 3 in column 1 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

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

Step 3: We'll start by adding the digits 0 & 5 in column 3 and get 5.

Step 4: We'll start by adding the digits 0 & 1 in column 4 and get 1.

So, 18 + 1503 = 1521.

