390240 ÷ 21 
The aim is to understand the frequency of 21 in 390240.

Advancing to step 1:
21 can be fit into 3 0 times, resulting in a remainder of 3.
The next digit of our result is 0. 
Result so far: 0.0
If we subtract 0 from 3, we get 3.
Bring next digit (9) of the dividend behind the 3 and repeat the process: 39 / 21

On to step 2:
The number 21 fits into 39 1 times, leaving a remainder of 18.
The next digit of our result is 1. 
Result so far: 1.0
If we take 21 away from 39, we end up with 18.
Grab the next digit (0) from the dividend, add it to 18, then carry on: 180 / 21

Going ahead to step 3:
180 divided by 21 is 8 with a remainder of 12.
The next digit of our result is 8. 
Result so far: 18.0
Subtract 168 from 180 to get 12.
Bring next digit (2) of the dividend behind the 12 and repeat the process: 122 / 21

Moving on to step 4:
When dividing 122 by 21, we get 5 with a remainder of 17.
The number 5 becomes the next digit in our result. 
Result so far: 185.0
If we subtract 105 from 122, we get 17.
Append the next digit (4) from the dividend to 17 and continue with: 174 / 21

Going ahead to step 5:
21 goes into 174 8 times with a remainder of 6.
The number 8 becomes the next digit in our result. 
Result so far: 1858.0
If we take 168 away from 174, we end up with 6.
Take the next digit (0) from the dividend and append it to 6, then repeat: 60 / 21

On to step 6:
When dividing 60 by 21, we get 2 with a remainder of 18.
Record the quotient 2 as the next digit in the result. 
Result so far: 18582.0
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.

Going ahead to step 7:
If we divide 180 by 21, we get 8 and a remainder of 12.
The next digit of our result is 8. 
Result so far: 18582.8
If we take 168 away from 180, we end up with 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 8:
120 divided by 21 is 5 with a remainder of 15.
Put 5 as the next digit of the answer. 
Result so far: 18582.85
Subtracting 105 from 120 leaves us 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.

Moving on to step 9:
21 goes into 150 7 times with a remainder of 3.
The next digit of our result is 7. 
Result so far: 18582.857
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.

Let's proceed to step 10:
30 divided by 21 is 1 with a remainder of 9.
Put 1 as the next digit of the answer. 
Result so far: 18582.8571
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 11:
21 goes into 90 4 times with a remainder of 6.
Record the quotient 4 as the next digit in the result. 
Result so far: 18582.85714
If we take 84 away from 90, we end up with 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Going ahead to step 12:
When dividing 60 by 21, we get 2 with a remainder of 18.
Record the quotient 2 as the next digit in the result. 
Result so far: 18582.857142
If we subtract 42 from 60, we get 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

Going ahead to step 13:
If we divide 180 by 21, we get 8 and a remainder of 12.
The next digit of our result is 8. 
Result so far: 18582.8571428
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.

Let's proceed to step 14:
120 divided by 21 is 5 with a remainder of 15.
Write down 5 as next digit of the result. 
Result so far: 18582.85714285
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.

Our division results in 18582.85714285 with a remaining 1.5e-08.