Here we go! We're going to add 3900 and 843 together.

Step 1: We'll start by adding the digits 0 & 3 in column 1 and get 3.

Step 2: We'll start by adding the digits 0 & 4 in column 2 and get 4.

Step 3: We'll start by adding the digits 9 & 8 in column 3 and get 17.
We'll write down the last digit 7 and carry the 1 to the next column.

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

So, 3900 + 843 = 4743.

