We look at the division of 63253786 by 21 
The aim is to understand the frequency of 21 in 63253786.

On to step 1:
When dividing 6 by 21, we get 0 with a remainder of 6.
The next digit of our result is 0. 
Result so far: 0.0
The remainder is 6 after subtracting 0 from 6.
Bring next digit (3) of the dividend behind the 6 and repeat the process: 63 / 21

Moving on to step 2:
The number 21 fits into 63 3 times, leaving a remainder of 0.
Use 3 as the next digit of our solution. 
Result so far: 3.0
If we subtract 63 from 63, we get 0.
Append the next digit (2) from the dividend to 0 and continue with: 2 / 21

On to step 3:
21 can be fit into 2 0 times, resulting in a remainder of 2.
Put 0 as the next digit of the answer. 
Result so far: 30.0
Subtract 0 from 2 to get 2.
Take the next digit (5) from the dividend and append it to 2, then repeat: 25 / 21

Going ahead to step 4:
When dividing 25 by 21, we get 1 with a remainder of 4.
Put 1 as the next digit of the answer. 
Result so far: 301.0
Subtract 21 from 25 to get 4.
Take the next digit (3) from the dividend and append it to 4, then repeat: 43 / 21

Moving on to step 5:
The number 21 fits into 43 2 times, leaving a remainder of 1.
Use 2 as the next digit of our solution. 
Result so far: 3012.0
Deduct 42 from 43 and we're left with 1.
Take the next digit (7) from the dividend and append it to 1, then repeat: 17 / 21

On to step 6:
If we divide 17 by 21, we get 0 and a remainder of 17.
Write down 0 as next digit of the result. 
Result so far: 30120.0
Subtract 0 from 17 to get 17.
Take the next digit (8) from the dividend and append it to 17, then repeat: 178 / 21

Going ahead to step 7:
The number 21 fits into 178 8 times, leaving a remainder of 10.
Write down 8 as next digit of the result. 
Result so far: 301208.0
If we subtract 168 from 178, we get 10.
Fetch the next digit (6) from the dividend, attach it to 10 and continue: 106 / 21

Advancing to step 8:
106 divided by 21 is 5 with a remainder of 1.
The next digit of our result is 5. 
Result so far: 3012085.0
Subtract 105 from 106 to get 1.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Let's proceed to step 9:
21 can be fit into 10 0 times, resulting in a remainder of 10.
Write down 0 as next digit of the result. 
Result so far: 3012085.0
Subtracting 0 from 10 leaves us with 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Going ahead to step 10:
The number 21 fits into 100 4 times, leaving a remainder of 16.
Record the quotient 4 as the next digit in the result. 
Result so far: 3012085.04
If we subtract 84 from 100, we get 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

On to step 11:
21 goes into 160 7 times with a remainder of 13.
Use 7 as the next digit of our solution. 
Result so far: 3012085.047
If we subtract 147 from 160, we get 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Going ahead to step 12:
21 can be fit into 130 6 times, resulting in a remainder of 4.
The number 6 becomes the next digit in our result. 
Result so far: 3012085.0476
Deduct 126 from 130 and we're left with 4.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

On to step 13:
21 goes into 40 1 times with a remainder of 19.
The number 1 becomes the next digit in our result. 
Result so far: 3012085.04761
The remainder is 19 after subtracting 21 from 40.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

Moving on to step 14:
21 goes into 190 9 times with a remainder of 1.
Put 9 as the next digit of the answer. 
Result so far: 3012085.047619
Deduct 189 from 190 and we're left with 1.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Going ahead to step 15:
If we divide 10 by 21, we get 0 and a remainder of 10.
Use 0 as the next digit of our solution. 
Result so far: 3012085.047619
Subtract 0 from 10 to get 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Advancing to step 16:
The number 21 fits into 100 4 times, leaving a remainder of 16.
Write down 4 as next digit of the result. 
Result so far: 3012085.04761904
Deduct 84 from 100 and we're left with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

The final result is 3012085.04761904 with a remainder of 1.6e-08.