420484 ÷ 21 
The aim is to understand the frequency of 21 in 420484.

Going ahead to step 1:
The number 21 fits into 4 0 times, leaving a remainder of 4.
Put 0 as the next digit of the answer. 
Result so far: 0.0
Deduct 0 from 4 and we're left with 4.
Take the next digit (2) from the dividend and append it to 4, then repeat: 42 / 21

Let's proceed to step 2:
When dividing 42 by 21, we get 2 with a remainder of 0.
Record the quotient 2 as the next digit in the result. 
Result so far: 2.0
Deduct 42 from 42 and we're left with 0.
Grab the next digit (0) from the dividend, add it to 0, then carry on: 0 / 21

On to step 3:
21 can be fit into 0 0 times, resulting in a remainder of 0.
Write down 0 as next digit of the result. 
Result so far: 20.0
If we subtract 0 from 0, we get 0.
Take the next digit (4) from the dividend and append it to 0, then repeat: 4 / 21

Let's proceed to step 4:
If we divide 4 by 21, we get 0 and a remainder of 4.
The next digit of our result is 0. 
Result so far: 200.0
Deduct 0 from 4 and we're left with 4.
Append the next digit (8) from the dividend to 4 and continue with: 48 / 21

Going ahead to step 5:
48 divided by 21 is 2 with a remainder of 6.
Use 2 as the next digit of our solution. 
Result so far: 2002.0
If we subtract 42 from 48, we get 6.
Bring next digit (4) of the dividend behind the 6 and repeat the process: 64 / 21

Going ahead to step 6:
64 divided by 21 is 3 with a remainder of 1.
Write down 3 as next digit of the result. 
Result so far: 20023.0
Deduct 63 from 64 and we're left 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.

Moving on to step 7:
10 divided by 21 is 0 with a remainder of 10.
Put 0 as the next digit of the answer. 
Result so far: 20023.0
Subtracting 0 from 10 leaves us 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 8:
21 can be fit into 100 4 times, resulting in a remainder of 16.
Put 4 as the next digit of the answer. 
Result so far: 20023.04
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 9:
21 goes into 160 7 times with a remainder of 13.
The number 7 becomes the next digit in our result. 
Result so far: 20023.047
The remainder is 13 after subtracting 147 from 160.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

On to step 10:
21 goes into 130 6 times with a remainder of 4.
Use 6 as the next digit of our solution. 
Result so far: 20023.0476
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 11:
If we divide 40 by 21, we get 1 and a remainder of 19.
Use 1 as the next digit of our solution. 
Result so far: 20023.04761
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.

On to step 12:
If we divide 190 by 21, we get 9 and a remainder of 1.
The number 9 becomes the next digit in our result. 
Result so far: 20023.047619
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.

Step 13:
When dividing 10 by 21, we get 0 with a remainder of 10.
The number 0 becomes the next digit in our result. 
Result so far: 20023.047619
Subtracting 0 from 10 leaves us 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.

Step 14:
100 divided by 21 is 4 with a remainder of 16.
The next digit of our result is 4. 
Result so far: 20023.04761904
If we subtract 84 from 100, we get 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

The final outcome is 20023.04761904, with a leftover of 1.6e-08.