Okay, we are tasked with adding 254 and 15. Let's begin!

Step 1: We'll start by adding the digits 4 & 5 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 2 & 0 in column 3 and get 2.

So, 254 + 15 = 269.

