We look at the division of 493903 by 21 
The aim is to understand the frequency of 21 in 493903.

Advancing to step 1:
When dividing 4 by 21, we get 0 with a remainder of 4.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Subtracting 0 from 4 leaves us with 4.
Include the next digit (9) from the dividend after 4, then repeat: 49 / 21

Step 2:
If we divide 49 by 21, we get 2 and a remainder of 7.
Put 2 as the next digit of the answer. 
Result so far: 2.0
If we subtract 42 from 49, we get 7.
Include the next digit (3) from the dividend after 7, then repeat: 73 / 21

On to step 3:
21 can be fit into 73 3 times, resulting in a remainder of 10.
Write down 3 as next digit of the result. 
Result so far: 23.0
If we subtract 63 from 73, we get 10.
Bring next digit (9) of the dividend behind the 10 and repeat the process: 109 / 21

Step 4:
21 can be fit into 109 5 times, resulting in a remainder of 4.
Put 5 as the next digit of the answer. 
Result so far: 235.0
Subtracting 105 from 109 leaves us with 4.
Bring next digit (0) of the dividend behind the 4 and repeat the process: 40 / 21

Advancing to step 5:
When dividing 40 by 21, we get 1 with a remainder of 19.
The next digit of our result is 1. 
Result so far: 2351.0
If we subtract 21 from 40, we get 19.
Bring next digit (3) of the dividend behind the 19 and repeat the process: 193 / 21

On to step 6:
If we divide 193 by 21, we get 9 and a remainder of 4.
Write down 9 as next digit of the result. 
Result so far: 23519.0
Deduct 189 from 193 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.

Moving on to step 7:
When dividing 40 by 21, we get 1 with a remainder of 19.
Use 1 as the next digit of our solution. 
Result so far: 23519.1
Subtract 21 from 40 to get 19.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

Step 8:
When dividing 190 by 21, we get 9 with a remainder of 1.
Record the quotient 9 as the next digit in the result. 
Result so far: 23519.19
Subtract 189 from 190 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.

Step 9:
21 goes into 10 0 times with a remainder of 10.
The number 0 becomes the next digit in our result. 
Result so far: 23519.19
Deduct 0 from 10 and we're left 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.

On to step 10:
21 goes into 100 4 times with a remainder of 16.
Write down 4 as next digit of the result. 
Result so far: 23519.1904
The remainder is 16 after subtracting 84 from 100.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Let's proceed to step 11:
The number 21 fits into 160 7 times, leaving a remainder of 13.
Write down 7 as next digit of the result. 
Result so far: 23519.19047
Deduct 147 from 160 and we're left with 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:
When dividing 130 by 21, we get 6 with a remainder of 4.
Put 6 as the next digit of the answer. 
Result so far: 23519.190476
Subtracting 126 from 130 leaves us 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.

Let's proceed 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: 23519.1904761
Subtracting 21 from 40 leaves us with 19.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

On to step 14:
21 goes into 190 9 times with a remainder of 1.
Write down 9 as next digit of the result. 
Result so far: 23519.19047619
Subtracting 189 from 190 leaves us 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.

After the division, we end up with 23519.19047619 and a remainder of 1e-09.