Okay, we are given 4090 and 7812. Let's add them up step by step.

Step 1: We'll start by adding the digits 0 & 2 in column 1 and get 2.

Step 2: We'll start by adding the digits 9 & 1 in column 2 and get 10.
We'll write down the last digit 0 and carry the 1 to the next column.

Step 3: We'll start by adding the digits 0 & 8 in column 3 and get 9.

Step 4: We'll start by adding the digits 4 & 7 in column 4 and get 11.
We'll write down the last digit 1 and carry the 1 to the next column.

So, 4090 + 7812 = 11902.

