407633 ÷ 21 
The aim is to understand the frequency of 21 in 407633.

Going ahead to step 1:
When dividing 4 by 21, we get 0 with a remainder of 4.
Write down 0 as next digit of the result. 
Result so far: 0.0
The remainder is 4 after subtracting 0 from 4.
Append the next digit (0) from the dividend to 4 and continue with: 40 / 21

Going ahead to step 2:
The number 21 fits into 40 1 times, leaving a remainder of 19.
The number 1 becomes the next digit in our result. 
Result so far: 1.0
Deduct 21 from 40 and we're left with 19.
Bring next digit (7) of the dividend behind the 19 and repeat the process: 197 / 21

On to step 3:
When dividing 197 by 21, we get 9 with a remainder of 8.
Record the quotient 9 as the next digit in the result. 
Result so far: 19.0
The remainder is 8 after subtracting 189 from 197.
Bring next digit (6) of the dividend behind the 8 and repeat the process: 86 / 21

Let's proceed to step 4:
When dividing 86 by 21, we get 4 with a remainder of 2.
Write down 4 as next digit of the result. 
Result so far: 194.0
If we subtract 84 from 86, we get 2.
Take the next digit (3) from the dividend and append it to 2, then repeat: 23 / 21

Advancing to step 5:
The number 21 fits into 23 1 times, leaving a remainder of 2.
Put 1 as the next digit of the answer. 
Result so far: 1941.0
Subtracting 21 from 23 leaves us with 2.
Bring next digit (3) of the dividend behind the 2 and repeat the process: 23 / 21

Step 6:
When dividing 23 by 21, we get 1 with a remainder of 2.
Put 1 as the next digit of the answer. 
Result so far: 19411.0
Subtracting 21 from 23 leaves us with 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Advancing to step 7:
21 can be fit into 20 0 times, resulting in a remainder of 20.
Record the quotient 0 as the next digit in the result. 
Result so far: 19411.0
Subtracting 0 from 20 leaves us with 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

Advancing to step 8:
When dividing 200 by 21, we get 9 with a remainder of 11.
Record the quotient 9 as the next digit in the result. 
Result so far: 19411.09
If we take 189 away from 200, we end up with 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Advancing to step 9:
If we divide 110 by 21, we get 5 and a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 19411.095
If we take 105 away from 110, we end up with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Moving on to step 10:
50 divided by 21 is 2 with a remainder of 8.
The number 2 becomes the next digit in our result. 
Result so far: 19411.0952
The remainder is 8 after subtracting 42 from 50.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

On to step 11:
21 can be fit into 80 3 times, resulting in a remainder of 17.
Use 3 as the next digit of our solution. 
Result so far: 19411.09523
Subtracting 63 from 80 leaves us with 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Advancing to step 12:
170 divided by 21 is 8 with a remainder of 2.
Write down 8 as next digit of the result. 
Result so far: 19411.095238
Subtract 168 from 170 to get 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Going ahead to step 13:
The number 21 fits into 20 0 times, leaving a remainder of 20.
Write down 0 as next digit of the result. 
Result so far: 19411.095238
Subtracting 0 from 20 leaves us with 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

On to step 14:
When dividing 200 by 21, we get 9 with a remainder of 11.
The next digit of our result is 9. 
Result so far: 19411.09523809
The remainder is 11 after subtracting 189 from 200.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

After the division, we end up with 19411.09523809 and a remainder of 1.1e-08.