Okay, we are tasked with adding 123 and 392. Let's begin!

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

Step 2: We'll start by adding the digits 2 & 9 in column 2 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

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

So, 123 + 392 = 515.

