We've got two numbers: 3857 and 2815. Let's find their sum.

Step 1: We'll start by adding the digits 7 & 5 in column 1 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 5 & 1 in column 2 and get 7.

Step 3: We'll start by adding the digits 8 & 8 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, 3857 + 2815 = 6672.

