Okay, we are tasked with adding 3900 and 2785. Let's begin!

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 0 & 8 in column 2 and get 8.

Step 3: We'll start by adding the digits 9 & 7 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 3 & 2 in column 4 and get 6.

So, 3900 + 2785 = 6685.

