Okay, we are tasked with adding 4003 and 3540. Let's begin!

Step 1: We'll start by adding the digits 3 & 0 in column 1 and get 3.

Step 2: We'll start by adding the digits 0 & 4 in column 2 and get 4.

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 4 & 3 in column 4 and get 7.

So, 4003 + 3540 = 7543.

