Let's break this down. We're going to add 4013 and 2451 together.

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

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

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

Step 4: We'll start by adding the digits 4 & 2 in column 4 and get 6.

So, 4013 + 2451 = 6464.

