We look at the division of 396543 by 21 
The aim is to understand the frequency of 21 in 396543.

Going ahead 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
The remainder is 3 after subtracting 0 from 3.
Append the next digit (9) from the dividend to 3 and continue with: 39 / 21

On to step 2:
The number 21 fits into 39 1 times, leaving a remainder of 18.
Write down 1 as next digit of the result. 
Result so far: 1.0
If we subtract 21 from 39, we get 18.
Bring next digit (6) of the dividend behind the 18 and repeat the process: 186 / 21

Moving on to step 3:
21 goes into 186 8 times with a remainder of 18.
Put 8 as the next digit of the answer. 
Result so far: 18.0
Deduct 168 from 186 and we're left with 18.
Take the next digit (5) from the dividend and append it to 18, then repeat: 185 / 21

On to step 4:
If we divide 185 by 21, we get 8 and a remainder of 17.
Use 8 as the next digit of our solution. 
Result so far: 188.0
Subtracting 168 from 185 leaves us with 17.
Fetch the next digit (4) from the dividend, attach it to 17 and continue: 174 / 21

On to step 5:
174 divided by 21 is 8 with a remainder of 6.
Record the quotient 8 as the next digit in the result. 
Result so far: 1888.0
Subtracting 168 from 174 leaves us with 6.
Take the next digit (3) from the dividend and append it to 6, then repeat: 63 / 21

Let's proceed to step 6:
When dividing 63 by 21, we get 3 with a remainder of 0.
Use 3 as the next digit of our solution. 
Result so far: 18883.0
Deduct 63 from 63 and we're left with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Let's proceed to step 7:
If we divide 0 by 21, we get 0 and a remainder of 0.
Write down 0 as next digit of the result. 
Result so far: 18883.0
Subtracting 0 from 0 leaves us with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Step 8:
21 goes into 0 0 times with a remainder of 0.
Put 0 as the next digit of the answer. 
Result so far: 18883.0
If we take 0 away from 0, we end up with 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 outcome is 18883.0, with a leftover of 0.0.