We look at the division of 20781517 by 399 
The aim is to understand the frequency of 399 in 20781517.

Let's proceed to step 1:
399 can be fit into 2 0 times, resulting in a remainder of 2.
Put 0 as the next digit of the answer. 
Result so far: 0.0
The remainder is 2 after subtracting 0 from 2.
Bring next digit (0) of the dividend behind the 2 and repeat the process: 20 / 399

Let's proceed to step 2:
20 divided by 399 is 0 with a remainder of 20.
Write down 0 as next digit of the result. 
Result so far: 0.0
The remainder is 20 after subtracting 0 from 20.
Fetch the next digit (7) from the dividend, attach it to 20 and continue: 207 / 399

Let's proceed to step 3:
When dividing 207 by 399, we get 0 with a remainder of 207.
Write down 0 as next digit of the result. 
Result so far: 0.0
Deduct 0 from 207 and we're left with 207.
Include the next digit (8) from the dividend after 207, then repeat: 2078 / 399

Moving on to step 4:
If we divide 2078 by 399, we get 5 and a remainder of 83.
Write down 5 as next digit of the result. 
Result so far: 5.0
The remainder is 83 after subtracting 1995 from 2078.
Append the next digit (1) from the dividend to 83 and continue with: 831 / 399

Let's proceed to step 5:
When dividing 831 by 399, we get 2 with a remainder of 33.
The next digit of our result is 2. 
Result so far: 52.0
Subtract 798 from 831 to get 33.
Grab the next digit (5) from the dividend, add it to 33, then carry on: 335 / 399

Going ahead to step 6:
If we divide 335 by 399, we get 0 and a remainder of 335.
The next digit of our result is 0. 
Result so far: 520.0
If we subtract 0 from 335, we get 335.
Take the next digit (1) from the dividend and append it to 335, then repeat: 3351 / 399

Let's proceed to step 7:
If we divide 3351 by 399, we get 8 and a remainder of 159.
The next digit of our result is 8. 
Result so far: 5208.0
Subtracting 3192 from 3351 leaves us with 159.
Grab the next digit (7) from the dividend, add it to 159, then carry on: 1597 / 399

Step 8:
1597 divided by 399 is 4 with a remainder of 1.
Record the quotient 4 as the next digit in the result. 
Result so far: 52084.0
The remainder is 1 after subtracting 1596 from 1597.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Going ahead to step 9:
The number 399 fits into 10 0 times, leaving a remainder of 10.
The number 0 becomes the next digit in our result. 
Result so far: 52084.0
If we take 0 away from 10, we end up with 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Advancing to step 10:
The number 399 fits into 100 0 times, leaving a remainder of 100.
Record the quotient 0 as the next digit in the result. 
Result so far: 52084.0
If we take 0 away from 100, we end up with 100.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1000, and continue the process.

Our division results in 52084.0 with a remaining 0.1.