871259 ÷ 21 
The aim is to understand the frequency of 21 in 871259.

On to step 1:
8 divided by 21 is 0 with a remainder of 8.
The next digit of our result is 0. 
Result so far: 0.0
If we subtract 0 from 8, we get 8.
Append the next digit (7) from the dividend to 8 and continue with: 87 / 21

Going ahead to step 2:
21 goes into 87 4 times with a remainder of 3.
The number 4 becomes the next digit in our result. 
Result so far: 4.0
Subtracting 84 from 87 leaves us with 3.
Bring next digit (1) of the dividend behind the 3 and repeat the process: 31 / 21

On to step 3:
21 can be fit into 31 1 times, resulting in a remainder of 10.
Record the quotient 1 as the next digit in the result. 
Result so far: 41.0
Deduct 21 from 31 and we're left with 10.
Take the next digit (2) from the dividend and append it to 10, then repeat: 102 / 21

Going ahead to step 4:
102 divided by 21 is 4 with a remainder of 18.
Put 4 as the next digit of the answer. 
Result so far: 414.0
The remainder is 18 after subtracting 84 from 102.
Take the next digit (5) from the dividend and append it to 18, then repeat: 185 / 21

On to step 5:
The number 21 fits into 185 8 times, leaving a remainder of 17.
The number 8 becomes the next digit in our result. 
Result so far: 4148.0
The remainder is 17 after subtracting 168 from 185.
Take the next digit (9) from the dividend and append it to 17, then repeat: 179 / 21

Advancing to step 6:
If we divide 179 by 21, we get 8 and a remainder of 11.
Record the quotient 8 as the next digit in the result. 
Result so far: 41488.0
If we take 168 away from 179, 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.

Step 7:
The number 21 fits into 110 5 times, leaving a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 41488.5
Subtracting 105 from 110 leaves us 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.

Let's proceed to step 8:
When dividing 50 by 21, we get 2 with a remainder of 8.
Record the quotient 2 as the next digit in the result. 
Result so far: 41488.52
If we subtract 42 from 50, we get 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Let's proceed to step 9:
80 divided by 21 is 3 with a remainder of 17.
The next digit of our result is 3. 
Result so far: 41488.523
If we subtract 63 from 80, we get 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Moving on to step 10:
21 goes into 170 8 times with a remainder of 2.
Record the quotient 8 as the next digit in the result. 
Result so far: 41488.5238
The remainder is 2 after subtracting 168 from 170.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Step 11:
If we divide 20 by 21, we get 0 and a remainder of 20.
Record the quotient 0 as the next digit in the result. 
Result so far: 41488.5238
If we take 0 away from 20, we end up 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 12:
The number 21 fits into 200 9 times, leaving a remainder of 11.
The number 9 becomes the next digit in our result. 
Result so far: 41488.523809
Subtract 189 from 200 to get 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 13:
21 can be fit into 110 5 times, resulting in a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 41488.5238095
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.

Going ahead to step 14:
When dividing 50 by 21, we get 2 with a remainder of 8.
The next digit of our result is 2. 
Result so far: 41488.52380952
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.

Our division results in 41488.52380952 with a remaining 8e-09.