Okay, we are tasked with adding 6423 and 125. Let's begin!

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 2 & 2 in column 2 and get 4.

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

Step 4: We'll start by adding the digits 6 & 0 in column 4 and get 6.

So, 6423 + 125 = 6548.

