209564 divided by 21 
The aim is to understand the frequency of 21 in 209564.

On to step 1:
21 goes into 2 0 times with a remainder of 2.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtracting 0 from 2 leaves us with 2.
Grab the next digit (0) from the dividend, add it to 2, then carry on: 20 / 21

Let's proceed to step 2:
21 goes into 20 0 times with a remainder of 20.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we take 0 away from 20, we end up with 20.
Take the next digit (9) from the dividend and append it to 20, then repeat: 209 / 21

On to step 3:
21 goes into 209 9 times with a remainder of 20.
Write down 9 as next digit of the result. 
Result so far: 9.0
The remainder is 20 after subtracting 189 from 209.
Grab the next digit (5) from the dividend, add it to 20, then carry on: 205 / 21

On to step 4:
When dividing 205 by 21, we get 9 with a remainder of 16.
Record the quotient 9 as the next digit in the result. 
Result so far: 99.0
Subtract 189 from 205 to get 16.
Append the next digit (6) from the dividend to 16 and continue with: 166 / 21

On to step 5:
When dividing 166 by 21, we get 7 with a remainder of 19.
Use 7 as the next digit of our solution. 
Result so far: 997.0
If we subtract 147 from 166, we get 19.
Include the next digit (4) from the dividend after 19, then repeat: 194 / 21

Advancing to step 6:
21 goes into 194 9 times with a remainder of 5.
Record the quotient 9 as the next digit in the result. 
Result so far: 9979.0
Deduct 189 from 194 and we're left with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Going ahead to step 7:
21 goes into 50 2 times with a remainder of 8.
The next digit of our result is 2. 
Result so far: 9979.2
Subtract 42 from 50 to get 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Moving on to step 8:
When dividing 80 by 21, we get 3 with a remainder of 17.
Record the quotient 3 as the next digit in the result. 
Result so far: 9979.23
If we take 63 away from 80, we end up with 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Moving on to step 9:
21 can be fit into 170 8 times, resulting in a remainder of 2.
Put 8 as the next digit of the answer. 
Result so far: 9979.238
If we take 168 away from 170, we end up with 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Going ahead to step 10:
20 divided by 21 is 0 with a remainder of 20.
Use 0 as the next digit of our solution. 
Result so far: 9979.238
Subtracting 0 from 20 leaves us with 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

Moving on to step 11:
The number 21 fits into 200 9 times, leaving a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 9979.23809
If we subtract 189 from 200, we get 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

On to step 12:
21 goes into 110 5 times with a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 9979.238095
Subtract 105 from 110 to get 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Going ahead to step 13:
If we divide 50 by 21, we get 2 and a remainder of 8.
The next digit of our result is 2. 
Result so far: 9979.2380952
If we subtract 42 from 50, we get 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Moving on to step 14:
When dividing 80 by 21, we get 3 with a remainder of 17.
The number 3 becomes the next digit in our result. 
Result so far: 9979.23809523
Subtract 63 from 80 to get 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

The quotient of the division is 9979.23809523, and the remainder is 1.7e-08.