We have numbers 6616 and 8421. Let's start adding them together.

Step 1: We'll start by adding the digits 6 & 1 in column 1 and get 7.

Step 2: We'll start by adding the digits 1 & 2 in column 2 and get 3.

Step 3: We'll start by adding the digits 6 & 4 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 6 & 8 in column 4 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

So, 6616 + 8421 = 15037.

