OK, let's do this. We've got 9950 and 6645 and we're adding them all together.

Step 1: We'll start by adding the digits 0 & 5 in column 1 and get 5.

Step 2: We'll start by adding the digits 5 & 4 in column 2 and get 9.

Step 3: We'll start by adding the digits 9 & 6 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, 9950 + 6645 = 16595.

