We look at the division of 235087 by 21 
The aim is to understand the frequency of 21 in 235087.

On to step 1:
When dividing 2 by 21, we get 0 with a remainder of 2.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Subtracting 0 from 2 leaves us with 2.
Take the next digit (3) from the dividend and append it to 2, then repeat: 23 / 21

On to step 2:
21 can be fit into 23 1 times, resulting in a remainder of 2.
Use 1 as the next digit of our solution. 
Result so far: 1.0
Deduct 21 from 23 and we're left with 2.
Append the next digit (5) from the dividend to 2 and continue with: 25 / 21

Advancing to step 3:
25 divided by 21 is 1 with a remainder of 4.
The next digit of our result is 1. 
Result so far: 11.0
Subtracting 21 from 25 leaves us with 4.
Bring next digit (0) of the dividend behind the 4 and repeat the process: 40 / 21

On to step 4:
21 can be fit into 40 1 times, resulting in a remainder of 19.
The next digit of our result is 1. 
Result so far: 111.0
If we subtract 21 from 40, we get 19.
Include the next digit (8) from the dividend after 19, then repeat: 198 / 21

Moving on to step 5:
When dividing 198 by 21, we get 9 with a remainder of 9.
The next digit of our result is 9. 
Result so far: 1119.0
If we take 189 away from 198, we end up with 9.
Take the next digit (7) from the dividend and append it to 9, then repeat: 97 / 21

Advancing to step 6:
The number 21 fits into 97 4 times, leaving a remainder of 13.
Record the quotient 4 as the next digit in the result. 
Result so far: 11194.0
Subtracting 84 from 97 leaves us 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.

Step 7:
The number 21 fits into 130 6 times, leaving a remainder of 4.
Put 6 as the next digit of the answer. 
Result so far: 11194.6
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.

Advancing to step 8:
40 divided by 21 is 1 with a remainder of 19.
The number 1 becomes the next digit in our result. 
Result so far: 11194.61
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.

On to step 9:
190 divided by 21 is 9 with a remainder of 1.
Put 9 as the next digit of the answer. 
Result so far: 11194.619
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.

Going ahead to step 10:
21 goes into 10 0 times with a remainder of 10.
The next digit of our result is 0. 
Result so far: 11194.619
If we subtract 0 from 10, we 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 11:
21 can be fit into 100 4 times, resulting in a remainder of 16.
Write down 4 as next digit of the result. 
Result so far: 11194.61904
If we take 84 away from 100, we end up 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.

Moving on to step 12:
160 divided by 21 is 7 with a remainder of 13.
The next digit of our result is 7. 
Result so far: 11194.619047
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.

Step 13:
130 divided by 21 is 6 with a remainder of 4.
Record the quotient 6 as the next digit in the result. 
Result so far: 11194.6190476
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.

Step 14:
When dividing 40 by 21, we get 1 with a remainder of 19.
Write down 1 as next digit of the result. 
Result so far: 11194.61904761
Deduct 21 from 40 and we're left 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.

After the division, we end up with 11194.61904761 and a remainder of 1.9e-08.