Not to worry, we've got 81 and 6420. Let's get to adding them!

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

Step 2: We'll start by adding the digits 8 & 2 in column 2 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 0 & 4 in column 3 and get 5.

Step 4: We'll start by adding the digits 0 & 6 in column 4 and get 6.

So, 81 + 6420 = 6501.

