We're dividing 621712 by 21 
The aim is to understand the frequency of 21 in 621712.

Let's proceed to step 1:
The number 21 fits into 6 0 times, leaving a remainder of 6.
The next digit of our result is 0. 
Result so far: 0.0
The remainder is 6 after subtracting 0 from 6.
Fetch the next digit (2) from the dividend, attach it to 6 and continue: 62 / 21

On to step 2:
21 can be fit into 62 2 times, resulting in a remainder of 20.
The number 2 becomes the next digit in our result. 
Result so far: 2.0
Subtract 42 from 62 to get 20.
Append the next digit (1) from the dividend to 20 and continue with: 201 / 21

Step 3:
When dividing 201 by 21, we get 9 with a remainder of 12.
Put 9 as the next digit of the answer. 
Result so far: 29.0
Subtracting 189 from 201 leaves us with 12.
Append the next digit (7) from the dividend to 12 and continue with: 127 / 21

Going ahead to step 4:
If we divide 127 by 21, we get 6 and a remainder of 1.
Write down 6 as next digit of the result. 
Result so far: 296.0
Deduct 126 from 127 and we're left with 1.
Include the next digit (1) from the dividend after 1, then repeat: 11 / 21

Let's proceed to step 5:
21 goes into 11 0 times with a remainder of 11.
Record the quotient 0 as the next digit in the result. 
Result so far: 2960.0
If we take 0 away from 11, we end up with 11.
Take the next digit (2) from the dividend and append it to 11, then repeat: 112 / 21

Step 6:
112 divided by 21 is 5 with a remainder of 7.
Put 5 as the next digit of the answer. 
Result so far: 29605.0
The remainder is 7 after subtracting 105 from 112.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Advancing to step 7:
When dividing 70 by 21, we get 3 with a remainder of 7.
The next digit of our result is 3. 
Result so far: 29605.3
The remainder is 7 after subtracting 63 from 70.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Going ahead to step 8:
The number 21 fits into 70 3 times, leaving a remainder of 7.
Put 3 as the next digit of the answer. 
Result so far: 29605.33
Subtracting 63 from 70 leaves us with 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

The final result is 29605.33 with a remainder of 0.007.