878879 ÷ 21 
The aim is to understand the frequency of 21 in 878879.

Advancing 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 (7) from the dividend, add it to 8, then carry on: 87 / 21

On to step 2:
21 goes into 87 4 times with a remainder of 3.
The next digit of our result is 4. 
Result so far: 4.0
Subtract 84 from 87 to get 3.
Include the next digit (8) from the dividend after 3, then repeat: 38 / 21

On to step 3:
21 can be fit into 38 1 times, resulting in a remainder of 17.
The next digit of our result is 1. 
Result so far: 41.0
If we subtract 21 from 38, we get 17.
Include the next digit (8) from the dividend after 17, then repeat: 178 / 21

Going ahead to step 4:
21 can be fit into 178 8 times, resulting in a remainder of 10.
The number 8 becomes the next digit in our result. 
Result so far: 418.0
Subtracting 168 from 178 leaves us with 10.
Append the next digit (7) from the dividend to 10 and continue with: 107 / 21

Going ahead to step 5:
21 goes into 107 5 times with a remainder of 2.
Record the quotient 5 as the next digit in the result. 
Result so far: 4185.0
Subtracting 105 from 107 leaves us with 2.
Grab the next digit (9) from the dividend, add it to 2, then carry on: 29 / 21

Advancing to step 6:
29 divided by 21 is 1 with a remainder of 8.
Record the quotient 1 as the next digit in the result. 
Result so far: 41851.0
If we take 21 away from 29, we end up with 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Advancing to step 7:
When dividing 80 by 21, we get 3 with a remainder of 17.
The next digit of our result is 3. 
Result so far: 41851.3
Deduct 63 from 80 and we're left with 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 8:
21 can be fit into 170 8 times, resulting in a remainder of 2.
Put 8 as the next digit of the answer. 
Result so far: 41851.38
Subtract 168 from 170 to get 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Going ahead to step 9:
20 divided by 21 is 0 with a remainder of 20.
The number 0 becomes the next digit in our result. 
Result so far: 41851.38
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.

Moving on to step 10:
21 goes into 200 9 times with a remainder of 11.
Write down 9 as next digit of the result. 
Result so far: 41851.3809
If we take 189 away from 200, 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.

Let's proceed to step 11:
When dividing 110 by 21, we get 5 with a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 41851.38095
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.

Let's proceed to step 12:
21 can be fit into 50 2 times, resulting in a remainder of 8.
The number 2 becomes the next digit in our result. 
Result so far: 41851.380952
Deduct 42 from 50 and we're left with 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Step 13:
When dividing 80 by 21, we get 3 with a remainder of 17.
Write down 3 as next digit of the result. 
Result so far: 41851.3809523
Subtract 63 from 80 to 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.

Advancing to step 14:
When dividing 170 by 21, we get 8 with a remainder of 2.
The next digit of our result is 8. 
Result so far: 41851.38095238
Subtracting 168 from 170 leaves us with 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

In conclusion, the division gives 41851.38095238 with a balance of 2e-09.