725423 divided by 21 
The aim is to understand the frequency of 21 in 725423.

On to step 1:
If we divide 7 by 21, we get 0 and a remainder of 7.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Subtract 0 from 7 to get 7.
Include the next digit (2) from the dividend after 7, then repeat: 72 / 21

Step 2:
72 divided by 21 is 3 with a remainder of 9.
The number 3 becomes the next digit in our result. 
Result so far: 3.0
Deduct 63 from 72 and we're left with 9.
Include the next digit (5) from the dividend after 9, then repeat: 95 / 21

Moving on to step 3:
95 divided by 21 is 4 with a remainder of 11.
Record the quotient 4 as the next digit in the result. 
Result so far: 34.0
Deduct 84 from 95 and we're left with 11.
Grab the next digit (4) from the dividend, add it to 11, then carry on: 114 / 21

Going ahead to step 4:
114 divided by 21 is 5 with a remainder of 9.
Use 5 as the next digit of our solution. 
Result so far: 345.0
Subtracting 105 from 114 leaves us with 9.
Bring next digit (2) of the dividend behind the 9 and repeat the process: 92 / 21

On to step 5:
If we divide 92 by 21, we get 4 and a remainder of 8.
Use 4 as the next digit of our solution. 
Result so far: 3454.0
If we take 84 away from 92, we end up with 8.
Bring next digit (3) of the dividend behind the 8 and repeat the process: 83 / 21

Moving on to step 6:
21 can be fit into 83 3 times, resulting in a remainder of 20.
Put 3 as the next digit of the answer. 
Result so far: 34543.0
Subtract 63 from 83 to 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.

Moving on to step 7:
21 goes into 200 9 times with a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 34543.9
Subtract 189 from 200 to get 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Let's proceed to step 8:
110 divided by 21 is 5 with a remainder of 5.
The number 5 becomes the next digit in our result. 
Result so far: 34543.95
If we take 105 away from 110, we end up 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 9:
When dividing 50 by 21, we get 2 with a remainder of 8.
Record the quotient 2 as the next digit in the result. 
Result so far: 34543.952
Subtracting 42 from 50 leaves us with 8.
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 10:
80 divided by 21 is 3 with a remainder of 17.
The next digit of our result is 3. 
Result so far: 34543.9523
Deduct 63 from 80 and we're left with 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 11:
21 goes into 170 8 times with a remainder of 2.
The number 8 becomes the next digit in our result. 
Result so far: 34543.95238
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.

Moving on to step 12:
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: 34543.95238
The remainder is 20 after subtracting 0 from 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 13:
When dividing 200 by 21, we get 9 with a remainder of 11.
Record the quotient 9 as the next digit in the result. 
Result so far: 34543.9523809
If we take 189 away from 200, we end up 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.

On to step 14:
The number 21 fits into 110 5 times, leaving a remainder of 5.
The number 5 becomes the next digit in our result. 
Result so far: 34543.95238095
If we subtract 105 from 110, we get 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Our division results in 34543.95238095 with a remaining 5e-09.