We divide 303897 by 21
The aim is to understand the frequency of 21 in 303897.

On to step 1:
The number 21 fits into 3 0 times, leaving a remainder of 3.
Write down 0 as next digit of the result. 
Result so far: 0.0
If we take 0 away from 3, we end up with 3.
Fetch the next digit (0) from the dividend, attach it to 3 and continue: 30 / 21

On to step 2:
If we divide 30 by 21, we get 1 and a remainder of 9.
The next digit of our result is 1. 
Result so far: 1.0
Deduct 21 from 30 and we're left with 9.
Include the next digit (3) from the dividend after 9, then repeat: 93 / 21

Moving on to step 3:
93 divided by 21 is 4 with a remainder of 9.
The next digit of our result is 4. 
Result so far: 14.0
If we take 84 away from 93, we end up with 9.
Take the next digit (8) from the dividend and append it to 9, then repeat: 98 / 21

Moving on to step 4:
98 divided by 21 is 4 with a remainder of 14.
The number 4 becomes the next digit in our result. 
Result so far: 144.0
Subtract 84 from 98 to get 14.
Bring next digit (9) of the dividend behind the 14 and repeat the process: 149 / 21

On to step 5:
The number 21 fits into 149 7 times, leaving a remainder of 2.
Use 7 as the next digit of our solution. 
Result so far: 1447.0
If we take 147 away from 149, we end up with 2.
Take the next digit (7) from the dividend and append it to 2, then repeat: 27 / 21

Moving on to step 6:
21 can be fit into 27 1 times, resulting in a remainder of 6.
The next digit of our result is 1. 
Result so far: 14471.0
Subtract 21 from 27 to get 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Advancing to step 7:
21 can be fit into 60 2 times, resulting in a remainder of 18.
The number 2 becomes the next digit in our result. 
Result so far: 14471.2
Deduct 42 from 60 and we're left 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.

Moving on to step 8:
180 divided by 21 is 8 with a remainder of 12.
Record the quotient 8 as the next digit in the result. 
Result so far: 14471.28
Subtract 168 from 180 to get 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Advancing to step 9:
21 can be fit into 120 5 times, resulting in a remainder of 15.
Use 5 as the next digit of our solution. 
Result so far: 14471.285
If we take 105 away from 120, we end up with 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Step 10:
If we divide 150 by 21, we get 7 and a remainder of 3.
Use 7 as the next digit of our solution. 
Result so far: 14471.2857
If we take 147 away from 150, we end up 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.

Let's proceed to step 11:
When dividing 30 by 21, we get 1 with a remainder of 9.
Put 1 as the next digit of the answer. 
Result so far: 14471.28571
Subtracting 21 from 30 leaves us 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.

Advancing to step 12:
21 goes into 90 4 times with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 14471.285714
Subtract 84 from 90 to get 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Advancing to step 13:
The number 21 fits into 60 2 times, leaving a remainder of 18.
Put 2 as the next digit of the answer. 
Result so far: 14471.2857142
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.

Step 14:
The number 21 fits into 180 8 times, leaving a remainder of 12.
The number 8 becomes the next digit in our result. 
Result so far: 14471.28571428
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.

After the division, we end up with 14471.28571428 and a remainder of 1.2e-08.