We're dividing 126072 by 21 
The aim is to understand the frequency of 21 in 126072.

Step 1:
If we divide 1 by 21, we get 0 and a remainder of 1.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtracting 0 from 1 leaves us with 1.
Take the next digit (2) from the dividend and append it to 1, then repeat: 12 / 21

Going ahead to step 2:
If we divide 12 by 21, we get 0 and a remainder of 12.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Deduct 0 from 12 and we're left with 12.
Bring next digit (6) of the dividend behind the 12 and repeat the process: 126 / 21

Let's proceed to step 3:
126 divided by 21 is 6 with a remainder of 0.
Use 6 as the next digit of our solution. 
Result so far: 6.0
If we subtract 126 from 126, we get 0.
Append the next digit (0) from the dividend to 0 and continue with: 0 / 21

Going ahead to step 4:
21 can be fit into 0 0 times, resulting in a remainder of 0.
Record the quotient 0 as the next digit in the result. 
Result so far: 60.0
Subtracting 0 from 0 leaves us with 0.
Append the next digit (7) from the dividend to 0 and continue with: 7 / 21

Going ahead to step 5:
7 divided by 21 is 0 with a remainder of 7.
Put 0 as the next digit of the answer. 
Result so far: 600.0
Deduct 0 from 7 and we're left with 7.
Append the next digit (2) from the dividend to 7 and continue with: 72 / 21

Going ahead to step 6:
21 goes into 72 3 times with a remainder of 9.
Write down 3 as next digit of the result. 
Result so far: 6003.0
If we take 63 away from 72, 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.

Moving on to step 7:
If we divide 90 by 21, we get 4 and a remainder of 6.
The next digit of our result is 4. 
Result so far: 6003.4
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.

Step 8:
The number 21 fits into 60 2 times, leaving a remainder of 18.
The next digit of our result is 2. 
Result so far: 6003.42
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.

Moving on to step 9:
21 can be fit into 180 8 times, resulting in a remainder of 12.
Put 8 as the next digit of the answer. 
Result so far: 6003.428
If we subtract 168 from 180, we get 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Advancing to step 10:
If we divide 120 by 21, we get 5 and a remainder of 15.
The next digit of our result is 5. 
Result so far: 6003.4285
Subtracting 105 from 120 leaves us 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.

Let's proceed to step 11:
The number 21 fits into 150 7 times, leaving a remainder of 3.
Record the quotient 7 as the next digit in the result. 
Result so far: 6003.42857
Subtract 147 from 150 to get 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

On to step 12:
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: 6003.428571
The remainder is 9 after subtracting 21 from 30.
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 13:
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: 6003.4285714
Subtract 84 from 90 to 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.

Step 14:
60 divided by 21 is 2 with a remainder of 18.
Write down 2 as next digit of the result. 
Result so far: 6003.42857142
Subtract 42 from 60 to get 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

The final result is 6003.42857142 with a remainder of 1.8e-08.