Let's divide 78878 by 21
The aim is to understand the frequency of 21 in 78878.

On to step 1:
When dividing 7 by 21, we get 0 with a remainder of 7.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Subtract 0 from 7 to get 7.
Take the next digit (8) from the dividend and append it to 7, then repeat: 78 / 21

Step 2:
21 can be fit into 78 3 times, resulting in a remainder of 15.
The number 3 becomes the next digit in our result. 
Result so far: 3.0
Subtracting 63 from 78 leaves us with 15.
Append the next digit (8) from the dividend to 15 and continue with: 158 / 21

Let's proceed to step 3:
158 divided by 21 is 7 with a remainder of 11.
Write down 7 as next digit of the result. 
Result so far: 37.0
Subtract 147 from 158 to get 11.
Grab the next digit (7) from the dividend, add it to 11, then carry on: 117 / 21

Step 4:
If we divide 117 by 21, we get 5 and a remainder of 12.
Put 5 as the next digit of the answer. 
Result so far: 375.0
Subtracting 105 from 117 leaves us with 12.
Fetch the next digit (8) from the dividend, attach it to 12 and continue: 128 / 21

Moving on to step 5:
When dividing 128 by 21, we get 6 with a remainder of 2.
The next digit of our result is 6. 
Result so far: 3756.0
Subtract 126 from 128 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.

Moving on to step 6:
When dividing 20 by 21, we get 0 with a remainder of 20.
Put 0 as the next digit of the answer. 
Result so far: 3756.0
If we subtract 0 from 20, we get 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 7:
When dividing 200 by 21, we get 9 with a remainder of 11.
The number 9 becomes the next digit in our result. 
Result so far: 3756.09
Deduct 189 from 200 and we're left 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 8:
If we divide 110 by 21, we get 5 and a remainder of 5.
Use 5 as the next digit of our solution. 
Result so far: 3756.095
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 9:
21 can be fit into 50 2 times, resulting in a remainder of 8.
The next digit of our result is 2. 
Result so far: 3756.0952
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.

Step 10:
The number 21 fits into 80 3 times, leaving a remainder of 17.
The next digit of our result is 3. 
Result so far: 3756.09523
The remainder is 17 after subtracting 63 from 80.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Going ahead to step 11:
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: 3756.095238
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.

Moving on to step 12:
21 can be fit into 20 0 times, resulting in a remainder of 20.
Record the quotient 0 as the next digit in the result. 
Result so far: 3756.095238
Deduct 0 from 20 and we're left 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.

Going ahead to step 13:
When dividing 200 by 21, we get 9 with a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 3756.09523809
Subtracting 189 from 200 leaves us 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.

After the division, we end up with 3756.09523809 and a remainder of 1.1e-08.