OK, let's do this. We've got 5112120102 and 6917908907 and we're adding them all together.

Step 1: We'll start by adding the digits 2 & 7 in column 1 and get 9.

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

Step 3: We'll start by adding the digits 1 & 9 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 0 & 8 in column 4 and get 9.

Step 5: We'll start by adding the digits 2 & 0 in column 5 and get 2.

Step 6: We'll start by adding the digits 1 & 9 in column 6 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 7: We'll start by adding the digits 2 & 7 in column 7 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 8: We'll start by adding the digits 1 & 1 in column 8 and get 3.

Step 9: We'll start by adding the digits 1 & 9 in column 9 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 10: We'll start by adding the digits 5 & 6 in column 10 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

So, 5112120102 + 6917908907 = 12030029009.

