We look at the division of 65603755 by 21 
The aim is to understand the frequency of 21 in 65603755.

On to step 1:
If we divide 6 by 21, we get 0 and a remainder of 6.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we subtract 0 from 6, we get 6.
Take the next digit (5) from the dividend and append it to 6, then repeat: 65 / 21

Moving on to step 2:
When dividing 65 by 21, we get 3 with a remainder of 2.
The next digit of our result is 3. 
Result so far: 3.0
If we subtract 63 from 65, we get 2.
Bring next digit (6) of the dividend behind the 2 and repeat the process: 26 / 21

Let's proceed to step 3:
If we divide 26 by 21, we get 1 and a remainder of 5.
The number 1 becomes the next digit in our result. 
Result so far: 31.0
Subtracting 21 from 26 leaves us with 5.
Include the next digit (0) from the dividend after 5, then repeat: 50 / 21

Let's proceed to step 4:
21 goes into 50 2 times with a remainder of 8.
Put 2 as the next digit of the answer. 
Result so far: 312.0
The remainder is 8 after subtracting 42 from 50.
Append the next digit (3) from the dividend to 8 and continue with: 83 / 21

On to step 5:
The number 21 fits into 83 3 times, leaving a remainder of 20.
The number 3 becomes the next digit in our result. 
Result so far: 3123.0
Subtract 63 from 83 to get 20.
Take the next digit (7) from the dividend and append it to 20, then repeat: 207 / 21

On to step 6:
When dividing 207 by 21, we get 9 with a remainder of 18.
Use 9 as the next digit of our solution. 
Result so far: 31239.0
Deduct 189 from 207 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 7:
The number 21 fits into 185 8 times, leaving a remainder of 17.
The number 8 becomes the next digit in our result. 
Result so far: 312398.0
Subtract 168 from 185 to get 17.
Append the next digit (5) from the dividend to 17 and continue with: 175 / 21

Step 8:
When dividing 175 by 21, we get 8 with a remainder of 7.
Record the quotient 8 as the next digit in the result. 
Result so far: 3123988.0
If we subtract 168 from 175, we get 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Moving on to step 9:
21 goes into 70 3 times with a remainder of 7.
Record the quotient 3 as the next digit in the result. 
Result so far: 3123988.3
If we take 63 away from 70, we end up with 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Going ahead to step 10:
When dividing 70 by 21, we get 3 with a remainder of 7.
Write down 3 as next digit of the result. 
Result so far: 3123988.33
If we subtract 63 from 70, we get 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

The final result is 3123988.33 with a remainder of 0.007.