OK, let's do this. We've got 6401394040 and 4406792592 and we're adding them all together.

Step 1: We'll start by adding the digits 0 & 2 in column 1 and get 2.

Step 2: We'll start by adding the digits 4 & 9 in column 2 and get 13.
We'll write down the last digit 3 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 0 & 5 in column 3 and get 6.

Step 4: We'll start by adding the digits 4 & 2 in column 4 and get 6.

Step 5: We'll start by adding the digits 9 & 9 in column 5 and get 18.
We'll write down the last digit 8 and carry the 1 to the next column.

Step 6: We'll start by adding the digits 3 & 7 in column 6 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

Step 7: We'll start by adding the digits 1 & 6 in column 7 and get 8.

Step 8: We'll start by adding the digits 0 & 0 in column 8 and get 0.

Step 9: We'll start by adding the digits 4 & 4 in column 9 and get 8.

Step 10: We'll start by adding the digits 6 & 4 in column 10 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 6401394040 + 4406792592 = 10808186632.

