816451 divided by 21 
The aim is to understand the frequency of 21 in 816451.

Let's proceed to step 1:
If we divide 8 by 21, we get 0 and a remainder of 8.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we subtract 0 from 8, we get 8.
Grab the next digit (1) from the dividend, add it to 8, then carry on: 81 / 21

On to step 2:
81 divided by 21 is 3 with a remainder of 18.
Write down 3 as next digit of the result. 
Result so far: 3.0
The remainder is 18 after subtracting 63 from 81.
Include the next digit (6) from the dividend after 18, then repeat: 186 / 21

Let's proceed to step 3:
The number 21 fits into 186 8 times, leaving a remainder of 18.
Record the quotient 8 as the next digit in the result. 
Result so far: 38.0
Subtract 168 from 186 to get 18.
Grab the next digit (4) from the dividend, add it to 18, then carry on: 184 / 21

Going ahead to step 4:
184 divided by 21 is 8 with a remainder of 16.
The next digit of our result is 8. 
Result so far: 388.0
Subtracting 168 from 184 leaves us with 16.
Fetch the next digit (5) from the dividend, attach it to 16 and continue: 165 / 21

On to step 5:
When dividing 165 by 21, we get 7 with a remainder of 18.
Write down 7 as next digit of the result. 
Result so far: 3887.0
Subtracting 147 from 165 leaves us with 18.
Fetch the next digit (1) from the dividend, attach it to 18 and continue: 181 / 21

Going ahead to step 6:
21 goes into 181 8 times with a remainder of 13.
The next digit of our result is 8. 
Result so far: 38878.0
Deduct 168 from 181 and we're left with 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Moving on to step 7:
21 can be fit into 130 6 times, resulting in a remainder of 4.
Write down 6 as next digit of the result. 
Result so far: 38878.6
The remainder is 4 after subtracting 126 from 130.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

Going ahead to step 8:
If we divide 40 by 21, we get 1 and a remainder of 19.
Record the quotient 1 as the next digit in the result. 
Result so far: 38878.61
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.

Advancing to step 9:
The number 21 fits into 190 9 times, leaving a remainder of 1.
The number 9 becomes the next digit in our result. 
Result so far: 38878.619
Subtract 189 from 190 to get 1.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Going ahead to step 10:
The number 21 fits into 10 0 times, leaving a remainder of 10.
Put 0 as the next digit of the answer. 
Result so far: 38878.619
If we take 0 away from 10, we end up 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.

Moving on to step 11:
21 goes into 100 4 times with a remainder of 16.
Put 4 as the next digit of the answer. 
Result so far: 38878.61904
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.

On to step 12:
If we divide 160 by 21, we get 7 and a remainder of 13.
Use 7 as the next digit of our solution. 
Result so far: 38878.619047
If we take 147 away from 160, we end up with 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Step 13:
21 goes into 130 6 times with a remainder of 4.
Put 6 as the next digit of the answer. 
Result so far: 38878.6190476
If we take 126 away from 130, we end up 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.

Moving on to step 14:
21 goes into 40 1 times with a remainder of 19.
The number 1 becomes the next digit in our result. 
Result so far: 38878.61904761
If we take 21 away from 40, we end up with 19.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

The final result is 38878.61904761 with a remainder of 1.9e-08.