Let's break this down. We're going to add 7454 and 8120 together.

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

Step 2: We'll start by adding the digits 5 & 2 in column 2 and get 7.

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 7 & 8 in column 4 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

So, 7454 + 8120 = 15574.

