Okay, let's tackle this math problem. We're starting with 20790 and subtracting 20415.

Step 1: We'll start by subtracting the digit 5 and the borrow 0 from 0 in column 1 and get -5.
We add -5 to 10 and get 5 as the first digit of the result.

Step 2: We'll start by subtracting the digit 1 and the borrow 1 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 4 and the borrow 0 from 7 in column 3 and get 3.
3 is the next digit of our result.

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

So, 20790 - 20415 = 375.