Let's break this down. We're going to add 9906 and 8140 together.

Step 1: We'll start by adding the digits 6 & 0 in column 1 and get 6.

Step 2: We'll start by adding the digits 0 & 4 in column 2 and get 4.

Step 3: We'll start by adding the digits 9 & 1 in column 3 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 9 & 8 in column 4 and get 18.
We'll write down the last digit 8 and carry the 1 to the next column.

So, 9906 + 8140 = 18046.

