Let's break this down. We're going to add 240 and 303 together.

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

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

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

So, 240 + 303 = 543.

