We look at the division of 423582 by 21 
The aim is to understand the frequency of 21 in 423582.

Let's proceed to step 1:
4 divided by 21 is 0 with a remainder of 4.
Write down 0 as next digit of the result. 
Result so far: 0.0
Deduct 0 from 4 and we're left with 4.
Include the next digit (2) from the dividend after 4, then repeat: 42 / 21

Going ahead to step 2:
42 divided by 21 is 2 with a remainder of 0.
Use 2 as the next digit of our solution. 
Result so far: 2.0
If we subtract 42 from 42, we get 0.
Include the next digit (3) from the dividend after 0, then repeat: 3 / 21

Advancing to step 3:
21 goes into 3 0 times with a remainder of 3.
Use 0 as the next digit of our solution. 
Result so far: 20.0
Subtract 0 from 3 to get 3.
Append the next digit (5) from the dividend to 3 and continue with: 35 / 21

Going ahead to step 4:
If we divide 35 by 21, we get 1 and a remainder of 14.
The next digit of our result is 1. 
Result so far: 201.0
If we subtract 21 from 35, we get 14.
Include the next digit (8) from the dividend after 14, then repeat: 148 / 21

Step 5:
21 can be fit into 148 7 times, resulting in a remainder of 1.
The next digit of our result is 7. 
Result so far: 2017.0
The remainder is 1 after subtracting 147 from 148.
Grab the next digit (2) from the dividend, add it to 1, then carry on: 12 / 21

Let's proceed to step 6:
21 goes into 12 0 times with a remainder of 12.
The number 0 becomes the next digit in our result. 
Result so far: 20170.0
Subtracting 0 from 12 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 7:
The number 21 fits into 120 5 times, leaving a remainder of 15.
Put 5 as the next digit of the answer. 
Result so far: 20170.5
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 8:
21 goes into 150 7 times with a remainder of 3.
Write down 7 as next digit of the result. 
Result so far: 20170.57
If we subtract 147 from 150, we 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.

Going ahead to step 9:
21 can be fit into 30 1 times, resulting in a remainder of 9.
Write down 1 as next digit of the result. 
Result so far: 20170.571
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 10:
When dividing 90 by 21, we get 4 with a remainder of 6.
Put 4 as the next digit of the answer. 
Result so far: 20170.5714
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.

Advancing to step 11:
When dividing 60 by 21, we get 2 with a remainder of 18.
Use 2 as the next digit of our solution. 
Result so far: 20170.57142
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.

Step 12:
21 goes into 180 8 times with a remainder of 12.
Put 8 as the next digit of the answer. 
Result so far: 20170.571428
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.

On to step 13:
When dividing 120 by 21, we get 5 with a remainder of 15.
Record the quotient 5 as the next digit in the result. 
Result so far: 20170.5714285
Subtract 105 from 120 to get 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Going ahead to step 14:
If we divide 150 by 21, we get 7 and a remainder of 3.
The number 7 becomes the next digit in our result. 
Result so far: 20170.57142857
If we subtract 147 from 150, we 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.

After the division, we end up with 20170.57142857 and a remainder of 3e-09.