We're dividing 903939 by 21 
The aim is to understand the frequency of 21 in 903939.

Advancing to step 1:
When dividing 9 by 21, we get 0 with a remainder of 9.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Deduct 0 from 9 and we're left with 9.
Append the next digit (0) from the dividend to 9 and continue with: 90 / 21

Moving on to step 2:
21 goes into 90 4 times with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 4.0
Deduct 84 from 90 and we're left with 6.
Include the next digit (3) from the dividend after 6, then repeat: 63 / 21

Step 3:
21 goes into 63 3 times with a remainder of 0.
Use 3 as the next digit of our solution. 
Result so far: 43.0
Subtract 63 from 63 to get 0.
Take the next digit (9) from the dividend and append it to 0, then repeat: 9 / 21

Moving on to step 4:
21 can be fit into 9 0 times, resulting in a remainder of 9.
Use 0 as the next digit of our solution. 
Result so far: 430.0
The remainder is 9 after subtracting 0 from 9.
Bring next digit (3) of the dividend behind the 9 and repeat the process: 93 / 21

Step 5:
The number 21 fits into 93 4 times, leaving a remainder of 9.
Use 4 as the next digit of our solution. 
Result so far: 4304.0
Subtract 84 from 93 to get 9.
Take the next digit (9) from the dividend and append it to 9, then repeat: 99 / 21

Advancing to step 6:
21 goes into 99 4 times with a remainder of 15.
Put 4 as the next digit of the answer. 
Result so far: 43044.0
If we take 84 away from 99, 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.

Going ahead to step 7:
150 divided by 21 is 7 with a remainder of 3.
The next digit of our result is 7. 
Result so far: 43044.7
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.

Step 8:
21 can be fit into 30 1 times, resulting in a remainder of 9.
Use 1 as the next digit of our solution. 
Result so far: 43044.71
If we take 21 away from 30, we end up 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.

On to step 9:
When dividing 90 by 21, we get 4 with a remainder of 6.
Record the quotient 4 as the next digit in the result. 
Result so far: 43044.714
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.

Let's proceed to step 10:
If we divide 60 by 21, we get 2 and a remainder of 18.
Use 2 as the next digit of our solution. 
Result so far: 43044.7142
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.

Let's proceed to step 11:
If we divide 180 by 21, we get 8 and a remainder of 12.
Write down 8 as next digit of the result. 
Result so far: 43044.71428
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 12:
The number 21 fits into 120 5 times, leaving a remainder of 15.
Put 5 as the next digit of the answer. 
Result so far: 43044.714285
The remainder is 15 after subtracting 105 from 120.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Moving on to step 13:
If we divide 150 by 21, we get 7 and a remainder of 3.
The number 7 becomes the next digit in our result. 
Result so far: 43044.7142857
Subtract 147 from 150 to get 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Advancing to step 14:
21 goes into 30 1 times with a remainder of 9.
The number 1 becomes the next digit in our result. 
Result so far: 43044.71428571
The remainder is 9 after subtracting 21 from 30.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

In conclusion, the division gives 43044.71428571 with a balance of 9e-09.