We look at the division of 509133 by 21 
The aim is to understand the frequency of 21 in 509133.

On to step 1:
21 can be fit into 5 0 times, resulting in a remainder of 5.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtracting 0 from 5 leaves us with 5.
Append the next digit (0) from the dividend to 5 and continue with: 50 / 21

Let's proceed to step 2:
21 can be fit into 50 2 times, resulting in a remainder of 8.
Record the quotient 2 as the next digit in the result. 
Result so far: 2.0
The remainder is 8 after subtracting 42 from 50.
Take the next digit (9) from the dividend and append it to 8, then repeat: 89 / 21

Let's proceed to step 3:
89 divided by 21 is 4 with a remainder of 5.
Record the quotient 4 as the next digit in the result. 
Result so far: 24.0
Subtract 84 from 89 to get 5.
Include the next digit (1) from the dividend after 5, then repeat: 51 / 21

Going ahead to step 4:
The number 21 fits into 51 2 times, leaving a remainder of 9.
The next digit of our result is 2. 
Result so far: 242.0
Deduct 42 from 51 and we're left with 9.
Fetch the next digit (3) from the dividend, attach it to 9 and continue: 93 / 21

Let's proceed to step 5:
21 can be fit into 93 4 times, resulting in a remainder of 9.
Record the quotient 4 as the next digit in the result. 
Result so far: 2424.0
Deduct 84 from 93 and we're left with 9.
Include the next digit (3) from the dividend after 9, then repeat: 93 / 21

Advancing to step 6:
If we divide 93 by 21, we get 4 and a remainder of 9.
Record the quotient 4 as the next digit in the result. 
Result so far: 24244.0
The remainder is 9 after subtracting 84 from 93.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Going ahead to step 7:
When dividing 90 by 21, we get 4 with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 24244.4
Subtract 84 from 90 to get 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Let's proceed to step 8:
If we divide 60 by 21, we get 2 and a remainder of 18.
The number 2 becomes the next digit in our result. 
Result so far: 24244.42
Subtract 42 from 60 to get 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

Let's proceed to step 9:
21 goes into 180 8 times with a remainder of 12.
The number 8 becomes the next digit in our result. 
Result so far: 24244.428
If we subtract 168 from 180, we get 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Going ahead to step 10:
120 divided by 21 is 5 with a remainder of 15.
The next digit of our result is 5. 
Result so far: 24244.4285
If we subtract 105 from 120, we get 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Going ahead to step 11:
If we divide 150 by 21, we get 7 and a remainder of 3.
The number 7 becomes the next digit in our result. 
Result so far: 24244.42857
The remainder is 3 after subtracting 147 from 150.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Moving on to step 12:
If we divide 30 by 21, we get 1 and a remainder of 9.
The number 1 becomes the next digit in our result. 
Result so far: 24244.428571
If we subtract 21 from 30, we get 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Moving on to step 13:
21 can be fit into 90 4 times, resulting in a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 24244.4285714
The remainder is 6 after subtracting 84 from 90.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Moving on to step 14:
When dividing 60 by 21, we get 2 with a remainder of 18.
The number 2 becomes the next digit in our result. 
Result so far: 24244.42857142
The remainder is 18 after subtracting 42 from 60.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

After the division, we end up with 24244.42857142 and a remainder of 1.8e-08.