We have numbers 4006 and 6661. Let's start adding them together.

Step 1: We'll start by adding the digits 6 & 1 in column 1 and get 7.

Step 2: We'll start by adding the digits 0 & 6 in column 2 and get 6.

Step 3: We'll start by adding the digits 0 & 6 in column 3 and get 6.

Step 4: We'll start by adding the digits 4 & 6 in column 4 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

So, 4006 + 6661 = 10667.

