Let's break this down. We're going to add 1603 and 1391 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 0 & 9 in column 2 and get 9.

Step 3: We'll start by adding the digits 6 & 3 in column 3 and get 9.

Step 4: We'll start by adding the digits 1 & 1 in column 4 and get 2.

So, 1603 + 1391 = 2994.

