We look at the division of 658338 by 21 
The aim is to understand the frequency of 21 in 658338.

Let's proceed to step 1:
21 goes into 6 0 times with a remainder of 6.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtract 0 from 6 to get 6.
Grab the next digit (5) from the dividend, add it to 6, then carry on: 65 / 21

On to step 2:
65 divided by 21 is 3 with a remainder of 2.
Write down 3 as next digit of the result. 
Result so far: 3.0
Subtracting 63 from 65 leaves us with 2.
Fetch the next digit (8) from the dividend, attach it to 2 and continue: 28 / 21

Advancing to step 3:
If we divide 28 by 21, we get 1 and a remainder of 7.
The number 1 becomes the next digit in our result. 
Result so far: 31.0
Deduct 21 from 28 and we're left with 7.
Append the next digit (3) from the dividend to 7 and continue with: 73 / 21

On to step 4:
21 goes into 73 3 times with a remainder of 10.
The number 3 becomes the next digit in our result. 
Result so far: 313.0
Subtract 63 from 73 to get 10.
Include the next digit (3) from the dividend after 10, then repeat: 103 / 21

Moving on to step 5:
The number 21 fits into 103 4 times, leaving a remainder of 19.
Write down 4 as next digit of the result. 
Result so far: 3134.0
If we take 84 away from 103, we end up with 19.
Take the next digit (8) from the dividend and append it to 19, then repeat: 198 / 21

Moving on to step 6:
21 can be fit into 198 9 times, resulting in a remainder of 9.
Record the quotient 9 as the next digit in the result. 
Result so far: 31349.0
Deduct 189 from 198 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.

Advancing to step 7:
The number 21 fits into 90 4 times, leaving a remainder of 6.
The next digit of our result is 4. 
Result so far: 31349.4
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 8:
When dividing 60 by 21, we get 2 with a remainder of 18.
The next digit of our result is 2. 
Result so far: 31349.42
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.

Let's proceed to step 9:
When dividing 180 by 21, we get 8 with a remainder of 12.
The next digit of our result is 8. 
Result so far: 31349.428
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.

Let's proceed to step 10:
When dividing 120 by 21, we get 5 with a remainder of 15.
Use 5 as the next digit of our solution. 
Result so far: 31349.4285
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.

Advancing to step 11:
If we divide 150 by 21, we get 7 and a remainder of 3.
Put 7 as the next digit of the answer. 
Result so far: 31349.42857
If we take 147 away from 150, we end up with 3.
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 12:
30 divided by 21 is 1 with a remainder of 9.
The number 1 becomes the next digit in our result. 
Result so far: 31349.428571
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 13:
90 divided by 21 is 4 with a remainder of 6.
The number 4 becomes the next digit in our result. 
Result so far: 31349.4285714
Deduct 84 from 90 and we're left 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 14:
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: 31349.42857142
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.

The final outcome is 31349.42857142, with a leftover of 1.8e-08.