872391 divided by 21 
The aim is to understand the frequency of 21 in 872391.

On to step 1:
8 divided by 21 is 0 with a remainder of 8.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we subtract 0 from 8, we get 8.
Include the next digit (7) from the dividend after 8, then repeat: 87 / 21

Going ahead to step 2:
If we divide 87 by 21, we get 4 and a remainder of 3.
Use 4 as the next digit of our solution. 
Result so far: 4.0
Deduct 84 from 87 and we're left with 3.
Bring next digit (2) of the dividend behind the 3 and repeat the process: 32 / 21

Moving on to step 3:
32 divided by 21 is 1 with a remainder of 11.
Record the quotient 1 as the next digit in the result. 
Result so far: 41.0
The remainder is 11 after subtracting 21 from 32.
Grab the next digit (3) from the dividend, add it to 11, then carry on: 113 / 21

Let's proceed to step 4:
21 can be fit into 113 5 times, resulting in a remainder of 8.
Use 5 as the next digit of our solution. 
Result so far: 415.0
Deduct 105 from 113 and we're left with 8.
Bring next digit (9) of the dividend behind the 8 and repeat the process: 89 / 21

Going ahead to step 5:
The number 21 fits into 89 4 times, leaving a remainder of 5.
The number 4 becomes the next digit in our result. 
Result so far: 4154.0
If we take 84 away from 89, we end up with 5.
Bring next digit (1) of the dividend behind the 5 and repeat the process: 51 / 21

Moving on to step 6:
21 can be fit into 51 2 times, resulting in a remainder of 9.
Record the quotient 2 as the next digit in the result. 
Result so far: 41542.0
If we take 42 away from 51, we end up with 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

On to step 7:
90 divided by 21 is 4 with a remainder of 6.
The next digit of our result is 4. 
Result so far: 41542.4
Subtracting 84 from 90 leaves us with 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

On to step 8:
The number 21 fits into 60 2 times, leaving a remainder of 18.
Write down 2 as next digit of the result. 
Result so far: 41542.42
If we take 42 away from 60, we end up with 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

On to step 9:
180 divided by 21 is 8 with a remainder of 12.
Use 8 as the next digit of our solution. 
Result so far: 41542.428
The remainder is 12 after subtracting 168 from 180.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Going ahead to step 10:
120 divided by 21 is 5 with a remainder of 15.
Write down 5 as next digit of the result. 
Result so far: 41542.4285
The remainder is 15 after subtracting 105 from 120.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Advancing to step 11:
If we divide 150 by 21, we get 7 and a remainder of 3.
The number 7 becomes the next digit in our result. 
Result so far: 41542.42857
Subtracting 147 from 150 leaves us with 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Advancing to step 12:
If we divide 30 by 21, we get 1 and a remainder of 9.
Put 1 as the next digit of the answer. 
Result so far: 41542.428571
If we take 21 away from 30, we end up with 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Going ahead to step 13:
If we divide 90 by 21, we get 4 and a remainder of 6.
Put 4 as the next digit of the answer. 
Result so far: 41542.4285714
Deduct 84 from 90 and we're left with 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Let's proceed to step 14:
The number 21 fits into 60 2 times, leaving a remainder of 18.
Use 2 as the next digit of our solution. 
Result so far: 41542.42857142
If we take 42 away from 60, we end up with 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

The quotient of the division is 41542.42857142, and the remainder is 1.8e-08.