OK, let's do this. We've got 1621 and 6770 and we're adding them all together.

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 2 & 7 in column 2 and get 9.

Step 3: We'll start by adding the digits 6 & 7 in column 3 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 1 & 6 in column 4 and get 8.

So, 1621 + 6770 = 8391.

