We've got two numbers: 9991 and 6517. 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 & 1 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 9 & 5 in column 3 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

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

So, 9991 + 6517 = 16508.

