We've got two numbers: 9900 and 39. Let's find their sum.

Step 1: We'll start by adding the digits 0 & 9 in column 1 and get 9.

Step 2: We'll start by adding the digits 0 & 3 in column 2 and get 3.

Step 3: We'll start by adding the digits 9 & 0 in column 3 and get 9.

Step 4: We'll start by adding the digits 9 & 0 in column 4 and get 9.

So, 9900 + 39 = 9939.

