828009 ÷ 21 
The aim is to understand the frequency of 21 in 828009.

On to step 1:
When dividing 8 by 21, we get 0 with a remainder of 8.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
The remainder is 8 after subtracting 0 from 8.
Grab the next digit (2) from the dividend, add it to 8, then carry on: 82 / 21

On to step 2:
21 goes into 82 3 times with a remainder of 19.
Use 3 as the next digit of our solution. 
Result so far: 3.0
The remainder is 19 after subtracting 63 from 82.
Bring next digit (8) of the dividend behind the 19 and repeat the process: 198 / 21

Moving on to step 3:
21 goes into 198 9 times with a remainder of 9.
Put 9 as the next digit of the answer. 
Result so far: 39.0
Subtract 189 from 198 to get 9.
Fetch the next digit (0) from the dividend, attach it to 9 and continue: 90 / 21

Step 4:
21 goes into 90 4 times with a remainder of 6.
The number 4 becomes the next digit in our result. 
Result so far: 394.0
Deduct 84 from 90 and we're left with 6.
Append the next digit (0) from the dividend to 6 and continue with: 60 / 21

Advancing to step 5:
If we divide 60 by 21, we get 2 and a remainder of 18.
The next digit of our result is 2. 
Result so far: 3942.0
Deduct 42 from 60 and we're left with 18.
Include the next digit (9) from the dividend after 18, then repeat: 189 / 21

Moving on to step 6:
21 goes into 189 9 times with a remainder of 0.
The next digit of our result is 9. 
Result so far: 39429.0
If we subtract 189 from 189, we get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Let's proceed to step 7:
When dividing 0 by 21, we get 0 with a remainder of 0.
The number 0 becomes the next digit in our result. 
Result so far: 39429.0
Deduct 0 from 0 and we're left with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Let's proceed to step 8:
The number 21 fits into 0 0 times, leaving a remainder of 0.
Record the quotient 0 as the next digit in the result. 
Result so far: 39429.0
Subtracting 0 from 0 leaves us with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Our division results in 39429.0 with a remaining 0.0.