No problem, let's work through this together. We're starting with 45991 and are subtracting 25120 from it.

Step 1: We'll start by subtracting the digit 0 and the borrow 0 from 1 in column 1 and get 1.
1 is the first digit of our result.

Step 2: We'll start by subtracting the digit 2 and the borrow 0 from 9 in column 2 and get 7.
7 is the next digit of our result.

Step 3: We'll start by subtracting the digit 1 and the borrow 0 from 9 in column 3 and get 8.
8 is the next digit of our result.

Step 4: We'll start by subtracting the digit 5 and the borrow 0 from 5 in column 4 and get 0.
0 is the next digit of our result.

Step 5: We'll start by subtracting the digit 2 and the borrow 0 from 4 in column 5 and get 2.
2 is the next digit of our result.

So, 45991 - 25120 = 20871.