We are presented with 5504 and 1373. Let's work out the sum.

Step 1: We'll start by adding the digits 4 & 3 in column 1 and get 7.

Step 2: We'll start by adding the digits 0 & 7 in column 2 and get 7.

Step 3: We'll start by adding the digits 5 & 3 in column 3 and get 8.

Step 4: We'll start by adding the digits 5 & 1 in column 4 and get 6.

So, 5504 + 1373 = 6877.

