412551 ÷ 21 
The aim is to understand the frequency of 21 in 412551.

Step 1:
4 divided by 21 is 0 with a remainder of 4.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtract 0 from 4 to get 4.
Take the next digit (1) from the dividend and append it to 4, then repeat: 41 / 21

Moving on to step 2:
41 divided by 21 is 1 with a remainder of 20.
Write down 1 as next digit of the result. 
Result so far: 1.0
Deduct 21 from 41 and we're left with 20.
Include the next digit (2) from the dividend after 20, then repeat: 202 / 21

Going ahead to step 3:
If we divide 202 by 21, we get 9 and a remainder of 13.
Use 9 as the next digit of our solution. 
Result so far: 19.0
Subtracting 189 from 202 leaves us with 13.
Grab the next digit (5) from the dividend, add it to 13, then carry on: 135 / 21

On to step 4:
The number 21 fits into 135 6 times, leaving a remainder of 9.
Write down 6 as next digit of the result. 
Result so far: 196.0
Subtracting 126 from 135 leaves us with 9.
Take the next digit (5) from the dividend and append it to 9, then repeat: 95 / 21

Going ahead to step 5:
21 goes into 95 4 times with a remainder of 11.
The number 4 becomes the next digit in our result. 
Result so far: 1964.0
If we take 84 away from 95, we end up with 11.
Fetch the next digit (1) from the dividend, attach it to 11 and continue: 111 / 21

Going ahead to step 6:
21 can be fit into 111 5 times, resulting in a remainder of 6.
The next digit of our result is 5. 
Result so far: 19645.0
Subtract 105 from 111 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.

Moving on to step 7:
When dividing 60 by 21, we get 2 with a remainder of 18.
The number 2 becomes the next digit in our result. 
Result so far: 19645.2
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.

Let's proceed to step 8:
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: 19645.28
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.

Going ahead to step 9:
21 goes into 120 5 times with a remainder of 15.
The next digit of our result is 5. 
Result so far: 19645.285
If we take 105 away from 120, we end up 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 10:
150 divided by 21 is 7 with a remainder of 3.
The number 7 becomes the next digit in our result. 
Result so far: 19645.2857
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:
If we divide 30 by 21, we get 1 and a remainder of 9.
The number 1 becomes the next digit in our result. 
Result so far: 19645.28571
Deduct 21 from 30 and we're left 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.

Step 12:
If we divide 90 by 21, we get 4 and a remainder of 6.
The number 4 becomes the next digit in our result. 
Result so far: 19645.285714
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.

Going ahead to step 13:
If we divide 60 by 21, we get 2 and a remainder of 18.
Record the quotient 2 as the next digit in the result. 
Result so far: 19645.2857142
Subtracting 42 from 60 leaves us 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.

Moving on to step 14:
21 can be fit into 180 8 times, resulting in a remainder of 12.
The next digit of our result is 8. 
Result so far: 19645.28571428
Deduct 168 from 180 and we're left 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.

After the division, we end up with 19645.28571428 and a remainder of 1.2e-08.