We look at the division of 512536 by 21 
The aim is to understand the frequency of 21 in 512536.

Moving on to step 1:
21 can be fit into 5 0 times, resulting in a remainder of 5.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Subtract 0 from 5 to get 5.
Bring next digit (1) of the dividend behind the 5 and repeat the process: 51 / 21

Going ahead to step 2:
When dividing 51 by 21, we get 2 with a remainder of 9.
The number 2 becomes the next digit in our result. 
Result so far: 2.0
Deduct 42 from 51 and we're left with 9.
Include the next digit (2) from the dividend after 9, then repeat: 92 / 21

Moving on to step 3:
The number 21 fits into 92 4 times, leaving a remainder of 8.
Use 4 as the next digit of our solution. 
Result so far: 24.0
The remainder is 8 after subtracting 84 from 92.
Take the next digit (5) from the dividend and append it to 8, then repeat: 85 / 21

On to step 4:
21 goes into 85 4 times with a remainder of 1.
Put 4 as the next digit of the answer. 
Result so far: 244.0
Subtracting 84 from 85 leaves us with 1.
Append the next digit (3) from the dividend to 1 and continue with: 13 / 21

Going ahead to step 5:
13 divided by 21 is 0 with a remainder of 13.
Record the quotient 0 as the next digit in the result. 
Result so far: 2440.0
Subtract 0 from 13 to get 13.
Take the next digit (6) from the dividend and append it to 13, then repeat: 136 / 21

Step 6:
When dividing 136 by 21, we get 6 with a remainder of 10.
The next digit of our result is 6. 
Result so far: 24406.0
If we take 126 away from 136, we end up 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.

Let's proceed to step 7:
When dividing 100 by 21, we get 4 with a remainder of 16.
Put 4 as the next digit of the answer. 
Result so far: 24406.4
Subtracting 84 from 100 leaves us 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.

Let's proceed to step 8:
21 can be fit into 160 7 times, resulting in a remainder of 13.
Record the quotient 7 as the next digit in the result. 
Result so far: 24406.47
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.

Let's proceed to step 9:
130 divided by 21 is 6 with a remainder of 4.
The number 6 becomes the next digit in our result. 
Result so far: 24406.476
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 10:
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: 24406.4761
If we subtract 21 from 40, we 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.

Going ahead to step 11:
When dividing 190 by 21, we get 9 with a remainder of 1.
Put 9 as the next digit of the answer. 
Result so far: 24406.47619
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 12:
21 goes into 10 0 times with a remainder of 10.
Write down 0 as next digit of the result. 
Result so far: 24406.47619
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.

Moving on to step 13:
If we divide 100 by 21, we get 4 and a remainder of 16.
The number 4 becomes the next digit in our result. 
Result so far: 24406.4761904
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.

On to step 14:
21 can be fit into 160 7 times, resulting in a remainder of 13.
Use 7 as the next digit of our solution. 
Result so far: 24406.47619047
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.

The final outcome is 24406.47619047, with a leftover of 1.3e-08.