We have numbers 8620 and 9899. Let's start adding them together.

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 2 & 9 in column 2 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 6 & 8 in column 3 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

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

So, 8620 + 9899 = 18519.

