We've got two numbers: 660 and 3961. Let's find their sum.

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 6 & 6 in column 2 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 6 & 9 in column 3 and get 16.
We'll write down the last digit 6 and carry the 1 to the next column.

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

So, 660 + 3961 = 4621.

