We're dividing 39951 by 21 
The aim is to understand the frequency of 21 in 39951.

Moving on to step 1:
When dividing 3 by 21, we get 0 with a remainder of 3.
Use 0 as the next digit of our solution. 
Result so far: 0.0
If we take 0 away from 3, we end up with 3.
Take the next digit (9) from the dividend and append it to 3, then repeat: 39 / 21

Moving on to step 2:
39 divided by 21 is 1 with a remainder of 18.
The number 1 becomes the next digit in our result. 
Result so far: 1.0
If we subtract 21 from 39, we get 18.
Fetch the next digit (9) from the dividend, attach it to 18 and continue: 189 / 21

Let's proceed to step 3:
189 divided by 21 is 9 with a remainder of 0.
Put 9 as the next digit of the answer. 
Result so far: 19.0
The remainder is 0 after subtracting 189 from 189.
Append the next digit (5) from the dividend to 0 and continue with: 5 / 21

Advancing to step 4:
When dividing 5 by 21, we get 0 with a remainder of 5.
The number 0 becomes the next digit in our result. 
Result so far: 190.0
The remainder is 5 after subtracting 0 from 5.
Append the next digit (1) from the dividend to 5 and continue with: 51 / 21

On to step 5:
When dividing 51 by 21, we get 2 with a remainder of 9.
Put 2 as the next digit of the answer. 
Result so far: 1902.0
If we take 42 away from 51, we end up with 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Let's proceed to step 6:
90 divided by 21 is 4 with a remainder of 6.
Put 4 as the next digit of the answer. 
Result so far: 1902.4
If we subtract 84 from 90, we get 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Advancing to step 7:
When dividing 60 by 21, we get 2 with a remainder of 18.
Put 2 as the next digit of the answer. 
Result so far: 1902.42
The remainder is 18 after subtracting 42 from 60.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

Going ahead to step 8:
21 can be fit into 180 8 times, resulting in a remainder of 12.
Record the quotient 8 as the next digit in the result. 
Result so far: 1902.428
Subtracting 168 from 180 leaves us with 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

On to step 9:
120 divided by 21 is 5 with a remainder of 15.
The number 5 becomes the next digit in our result. 
Result so far: 1902.4285
Deduct 105 from 120 and we're left with 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

On to step 10:
When dividing 150 by 21, we get 7 with a remainder of 3.
The number 7 becomes the next digit in our result. 
Result so far: 1902.42857
The remainder is 3 after subtracting 147 from 150.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Advancing to step 11:
The number 21 fits into 30 1 times, leaving a remainder of 9.
Record the quotient 1 as the next digit in the result. 
Result so far: 1902.428571
If we subtract 21 from 30, we get 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Going ahead to step 12:
The number 21 fits into 90 4 times, leaving a remainder of 6.
The number 4 becomes the next digit in our result. 
Result so far: 1902.4285714
Subtracting 84 from 90 leaves us with 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Let's proceed to step 13:
If we divide 60 by 21, we get 2 and a remainder of 18.
Write down 2 as next digit of the result. 
Result so far: 1902.42857142
If we take 42 away from 60, we end up with 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

Our division results in 1902.42857142 with a remaining 1.8e-08.