390726 ÷ 21 
The aim is to understand the frequency of 21 in 390726.

Advancing to step 1:
21 can be fit into 3 0 times, resulting in a remainder of 3.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Subtracting 0 from 3 leaves us with 3.
Fetch the next digit (9) from the dividend, attach it to 3 and continue: 39 / 21

On to step 2:
The number 21 fits into 39 1 times, leaving a remainder of 18.
Put 1 as the next digit of the answer. 
Result so far: 1.0
If we take 21 away from 39, we end up with 18.
Bring next digit (0) of the dividend behind the 18 and repeat the process: 180 / 21

On to step 3:
When dividing 180 by 21, we get 8 with a remainder of 12.
Write down 8 as next digit of the result. 
Result so far: 18.0
Subtracting 168 from 180 leaves us with 12.
Append the next digit (7) from the dividend to 12 and continue with: 127 / 21

Going ahead to step 4:
127 divided by 21 is 6 with a remainder of 1.
Write down 6 as next digit of the result. 
Result so far: 186.0
Deduct 126 from 127 and we're left with 1.
Take the next digit (2) from the dividend and append it to 1, then repeat: 12 / 21

Going ahead to step 5:
21 goes into 12 0 times with a remainder of 12.
The next digit of our result is 0. 
Result so far: 1860.0
If we take 0 away from 12, we end up with 12.
Grab the next digit (6) from the dividend, add it to 12, then carry on: 126 / 21

Step 6:
If we divide 126 by 21, we get 6 and a remainder of 0.
The next digit of our result is 6. 
Result so far: 18606.0
If we subtract 126 from 126, we get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Advancing to step 7:
21 goes into 0 0 times with a remainder of 0.
Use 0 as the next digit of our solution. 
Result so far: 18606.0
Subtract 0 from 0 to get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Moving on to step 8:
21 can be fit into 0 0 times, resulting in a remainder of 0.
Put 0 as the next digit of the answer. 
Result so far: 18606.0
The remainder is 0 after subtracting 0 from 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

The final result is 18606.0 with a remainder of 0.0.