723809 ÷ 21 
The aim is to understand the frequency of 21 in 723809.

On to step 1:
If we divide 7 by 21, we get 0 and a remainder of 7.
Write down 0 as next digit of the result. 
Result so far: 0.0
Subtract 0 from 7 to get 7.
Bring next digit (2) of the dividend behind the 7 and repeat the process: 72 / 21

Let's proceed to step 2:
21 can be fit into 72 3 times, resulting in a remainder of 9.
The number 3 becomes the next digit in our result. 
Result so far: 3.0
If we subtract 63 from 72, we get 9.
Take the next digit (3) from the dividend and append it to 9, then repeat: 93 / 21

Moving on to step 3:
The number 21 fits into 93 4 times, leaving a remainder of 9.
The next digit of our result is 4. 
Result so far: 34.0
The remainder is 9 after subtracting 84 from 93.
Append the next digit (8) from the dividend to 9 and continue with: 98 / 21

Let's proceed to step 4:
21 goes into 98 4 times with a remainder of 14.
Record the quotient 4 as the next digit in the result. 
Result so far: 344.0
If we take 84 away from 98, we end up with 14.
Grab the next digit (0) from the dividend, add it to 14, then carry on: 140 / 21

Let's proceed to step 5:
21 can be fit into 140 6 times, resulting in a remainder of 14.
Use 6 as the next digit of our solution. 
Result so far: 3446.0
The remainder is 14 after subtracting 126 from 140.
Take the next digit (9) from the dividend and append it to 14, then repeat: 149 / 21

Step 6:
When dividing 149 by 21, we get 7 with a remainder of 2.
The next digit of our result is 7. 
Result so far: 34467.0
If we take 147 away from 149, we end up with 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

On to step 7:
21 can be fit into 20 0 times, resulting in a remainder of 20.
The number 0 becomes the next digit in our result. 
Result so far: 34467.0
If we subtract 0 from 20, we get 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

Let's proceed to step 8:
If we divide 200 by 21, we get 9 and a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 34467.09
Subtracting 189 from 200 leaves us with 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Going ahead to step 9:
21 goes into 110 5 times with a remainder of 5.
The next digit of our result is 5. 
Result so far: 34467.095
Deduct 105 from 110 and we're left with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Let's proceed to step 10:
21 can be fit into 50 2 times, resulting in a remainder of 8.
Use 2 as the next digit of our solution. 
Result so far: 34467.0952
The remainder is 8 after subtracting 42 from 50.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Moving on to step 11:
If we divide 80 by 21, we get 3 and a remainder of 17.
Use 3 as the next digit of our solution. 
Result so far: 34467.09523
Subtract 63 from 80 to get 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Advancing to step 12:
170 divided by 21 is 8 with a remainder of 2.
The next digit of our result is 8. 
Result so far: 34467.095238
Deduct 168 from 170 and we're left with 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Step 13:
When dividing 20 by 21, we get 0 with a remainder of 20.
Put 0 as the next digit of the answer. 
Result so far: 34467.095238
If we take 0 away from 20, we end up with 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

On to step 14:
200 divided by 21 is 9 with a remainder of 11.
Write down 9 as next digit of the result. 
Result so far: 34467.09523809
Deduct 189 from 200 and we're left with 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

The final outcome is 34467.09523809, with a leftover of 1.1e-08.