521427 divided by 21 
The aim is to understand the frequency of 21 in 521427.

On to step 1:
When dividing 5 by 21, we get 0 with a remainder of 5.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we take 0 away from 5, we end up with 5.
Take the next digit (2) from the dividend and append it to 5, then repeat: 52 / 21

Let's proceed to step 2:
52 divided by 21 is 2 with a remainder of 10.
Write down 2 as next digit of the result. 
Result so far: 2.0
Deduct 42 from 52 and we're left with 10.
Fetch the next digit (1) from the dividend, attach it to 10 and continue: 101 / 21

Moving on to step 3:
21 goes into 101 4 times with a remainder of 17.
Write down 4 as next digit of the result. 
Result so far: 24.0
Subtract 84 from 101 to get 17.
Fetch the next digit (4) from the dividend, attach it to 17 and continue: 174 / 21

Going ahead to step 4:
21 goes into 174 8 times with a remainder of 6.
The next digit of our result is 8. 
Result so far: 248.0
Deduct 168 from 174 and we're left with 6.
Take the next digit (2) from the dividend and append it to 6, then repeat: 62 / 21

Moving on to step 5:
21 goes into 62 2 times with a remainder of 20.
The number 2 becomes the next digit in our result. 
Result so far: 2482.0
Subtracting 42 from 62 leaves us with 20.
Append the next digit (7) from the dividend to 20 and continue with: 207 / 21

Moving on to step 6:
If we divide 207 by 21, we get 9 and a remainder of 18.
The next digit of our result is 9. 
Result so far: 24829.0
Subtract 189 from 207 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.

Going ahead to step 7:
21 can be fit into 180 8 times, resulting in a remainder of 12.
Write down 8 as next digit of the result. 
Result so far: 24829.8
Subtract 168 from 180 to 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.

Moving on to step 8:
21 can be fit into 120 5 times, resulting in a remainder of 15.
Write down 5 as next digit of the result. 
Result so far: 24829.85
Subtracting 105 from 120 leaves us with 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

On to step 9:
The number 21 fits into 150 7 times, leaving a remainder of 3.
Use 7 as the next digit of our solution. 
Result so far: 24829.857
Subtracting 147 from 150 leaves us with 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Advancing to step 10:
The number 21 fits into 30 1 times, leaving a remainder of 9.
Record the quotient 1 as the next digit in the result. 
Result so far: 24829.8571
Subtracting 21 from 30 leaves us with 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 11:
21 can be fit into 90 4 times, resulting in a remainder of 6.
Record the quotient 4 as the next digit in the result. 
Result so far: 24829.85714
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.

Step 12:
21 can be fit into 60 2 times, resulting in a remainder of 18.
The number 2 becomes the next digit in our result. 
Result so far: 24829.857142
If we take 42 away from 60, we end up with 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 13:
21 can be fit into 180 8 times, resulting in a remainder of 12.
The number 8 becomes the next digit in our result. 
Result so far: 24829.8571428
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.

On to step 14:
21 can be fit into 120 5 times, resulting in a remainder of 15.
Write down 5 as next digit of the result. 
Result so far: 24829.85714285
Deduct 105 from 120 and we're left with 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

The final outcome is 24829.85714285, with a leftover of 1.5e-08.