We've got two numbers: 6991 and 9907. Let's find their sum.

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

Step 2: We'll start by adding the digits 9 & 0 in column 2 and get 9.

Step 3: We'll start by adding the digits 9 & 9 in column 3 and get 18.
We'll write down the last digit 8 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 6 & 9 in column 4 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

So, 6991 + 9907 = 16898.

