Okay, we are tasked with adding 25 and 2399. Let's begin!

Step 1: We'll start by adding the digits 5 & 9 in column 1 and get 14.
We'll write down the last digit 4 and carry the 1 to the next column.

Step 2: We'll start by adding the digits 2 & 9 in column 2 and get 12.
We'll write down the last digit 2 and carry the 1 to the next column.

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

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

So, 25 + 2399 = 2424.

