Let's break this down. We're going to add 5551 and 18 together.

Step 1: We'll start by adding the digits 1 & 8 in column 1 and get 9.

Step 2: We'll start by adding the digits 5 & 1 in column 2 and get 6.

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

Step 4: We'll start by adding the digits 5 & 0 in column 4 and get 5.

So, 5551 + 18 = 5569.

