We look at the division of 939172 by 21 
The aim is to understand the frequency of 21 in 939172.

Step 1:
9 divided by 21 is 0 with a remainder of 9.
Write down 0 as next digit of the result. 
Result so far: 0.0
If we take 0 away from 9, we end up with 9.
Bring next digit (3) of the dividend behind the 9 and repeat the process: 93 / 21

Advancing to step 2:
21 goes into 93 4 times with a remainder of 9.
The number 4 becomes the next digit in our result. 
Result so far: 4.0
Deduct 84 from 93 and we're left with 9.
Append the next digit (9) from the dividend to 9 and continue with: 99 / 21

Let's proceed to step 3:
21 goes into 99 4 times with a remainder of 15.
The number 4 becomes the next digit in our result. 
Result so far: 44.0
The remainder is 15 after subtracting 84 from 99.
Take the next digit (1) from the dividend and append it to 15, then repeat: 151 / 21

Moving on to step 4:
If we divide 151 by 21, we get 7 and a remainder of 4.
Write down 7 as next digit of the result. 
Result so far: 447.0
If we subtract 147 from 151, we get 4.
Bring next digit (7) of the dividend behind the 4 and repeat the process: 47 / 21

Let's proceed to step 5:
If we divide 47 by 21, we get 2 and a remainder of 5.
The next digit of our result is 2. 
Result so far: 4472.0
Subtract 42 from 47 to get 5.
Include the next digit (2) from the dividend after 5, then repeat: 52 / 21

Moving on to step 6:
52 divided by 21 is 2 with a remainder of 10.
Put 2 as the next digit of the answer. 
Result so far: 44722.0
Subtract 42 from 52 to get 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Moving on to step 7:
The number 21 fits into 100 4 times, leaving a remainder of 16.
The next digit of our result is 4. 
Result so far: 44722.4
Subtracting 84 from 100 leaves us with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Step 8:
21 can be fit into 160 7 times, resulting in a remainder of 13.
The next digit of our result is 7. 
Result so far: 44722.47
Deduct 147 from 160 and we're left with 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Advancing to step 9:
21 can be fit into 130 6 times, resulting in a remainder of 4.
Write down 6 as next digit of the result. 
Result so far: 44722.476
If we take 126 away from 130, we end up with 4.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

Going ahead to step 10:
21 goes into 40 1 times with a remainder of 19.
Record the quotient 1 as the next digit in the result. 
Result so far: 44722.4761
The remainder is 19 after subtracting 21 from 40.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

Let's proceed to step 11:
21 goes into 190 9 times with a remainder of 1.
Record the quotient 9 as the next digit in the result. 
Result so far: 44722.47619
The remainder is 1 after subtracting 189 from 190.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Advancing to step 12:
10 divided by 21 is 0 with a remainder of 10.
Use 0 as the next digit of our solution. 
Result so far: 44722.47619
If we take 0 away from 10, we end up with 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Let's proceed to step 13:
21 can be fit into 100 4 times, resulting in a remainder of 16.
The number 4 becomes the next digit in our result. 
Result so far: 44722.4761904
Subtracting 84 from 100 leaves us with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Step 14:
160 divided by 21 is 7 with a remainder of 13.
Record the quotient 7 as the next digit in the result. 
Result so far: 44722.47619047
If we subtract 147 from 160, we get 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

After the division, we end up with 44722.47619047 and a remainder of 1.3e-08.