OK, let's do this. We've got 7990 and 7941 and we're adding them all together.

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

Step 2: We'll start by adding the digits 9 & 4 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 9 & 9 in column 3 and get 19.
We'll write down the last digit 9 and carry the 1 to the next column.

Step 4: We'll start by adding the digits 7 & 7 in column 4 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

So, 7990 + 7941 = 15931.

