We look at the division of 688397 by 21 
The aim is to understand the frequency of 21 in 688397.

Let's proceed to step 1:
6 divided by 21 is 0 with a remainder of 6.
Put 0 as the next digit of the answer. 
Result so far: 0.0
If we take 0 away from 6, we end up with 6.
Take the next digit (8) from the dividend and append it to 6, then repeat: 68 / 21

Step 2:
21 can be fit into 68 3 times, resulting in a remainder of 5.
Use 3 as the next digit of our solution. 
Result so far: 3.0
The remainder is 5 after subtracting 63 from 68.
Fetch the next digit (8) from the dividend, attach it to 5 and continue: 58 / 21

On to step 3:
58 divided by 21 is 2 with a remainder of 16.
Write down 2 as next digit of the result. 
Result so far: 32.0
Subtracting 42 from 58 leaves us with 16.
Fetch the next digit (3) from the dividend, attach it to 16 and continue: 163 / 21

Moving on to step 4:
21 goes into 163 7 times with a remainder of 16.
Put 7 as the next digit of the answer. 
Result so far: 327.0
The remainder is 16 after subtracting 147 from 163.
Include the next digit (9) from the dividend after 16, then repeat: 169 / 21

Going ahead to step 5:
21 goes into 169 8 times with a remainder of 1.
Write down 8 as next digit of the result. 
Result so far: 3278.0
The remainder is 1 after subtracting 168 from 169.
Fetch the next digit (7) from the dividend, attach it to 1 and continue: 17 / 21

Advancing to step 6:
If we divide 17 by 21, we get 0 and a remainder of 17.
Use 0 as the next digit of our solution. 
Result so far: 32780.0
The remainder is 17 after subtracting 0 from 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Step 7:
If we divide 170 by 21, we get 8 and a remainder of 2.
The next digit of our result is 8. 
Result so far: 32780.8
Deduct 168 from 170 and we're left 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.

On to step 8:
The number 21 fits into 20 0 times, leaving a remainder of 20.
Write down 0 as next digit of the result. 
Result so far: 32780.8
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.

Let's proceed to step 9:
The number 21 fits into 200 9 times, leaving a remainder of 11.
Use 9 as the next digit of our solution. 
Result so far: 32780.809
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.

Moving on to step 10:
21 can be fit into 110 5 times, resulting in a remainder of 5.
Use 5 as the next digit of our solution. 
Result so far: 32780.8095
The remainder is 5 after subtracting 105 from 110.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Step 11:
21 goes into 50 2 times with a remainder of 8.
Put 2 as the next digit of the answer. 
Result so far: 32780.80952
Subtract 42 from 50 to 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.

On to step 12:
The number 21 fits into 80 3 times, leaving a remainder of 17.
Use 3 as the next digit of our solution. 
Result so far: 32780.809523
If we take 63 away from 80, we end up 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.

Advancing to step 13:
21 can be fit into 170 8 times, resulting in a remainder of 2.
The number 8 becomes the next digit in our result. 
Result so far: 32780.8095238
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.

Going ahead to step 14:
When dividing 20 by 21, we get 0 with a remainder of 20.
Use 0 as the next digit of our solution. 
Result so far: 32780.8095238
The remainder is 20 after subtracting 0 from 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

The final result is 32780.8095238 with a remainder of 2e-08.