Let's break this down. We're going to add 243 and 8115 together.

Step 1: We'll start by adding the digits 3 & 5 in column 1 and get 8.

Step 2: We'll start by adding the digits 4 & 1 in column 2 and get 5.

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

Step 4: We'll start by adding the digits 0 & 8 in column 4 and get 8.

So, 243 + 8115 = 8358.

