525981 ÷ 21 
The aim is to understand the frequency of 21 in 525981.

Advancing to step 1:
When dividing 5 by 21, we get 0 with a remainder of 5.
Write down 0 as next digit of the result. 
Result so far: 0.0
If we subtract 0 from 5, we get 5.
Grab the next digit (2) from the dividend, add it to 5, then carry on: 52 / 21

Going ahead to step 2:
52 divided by 21 is 2 with a remainder of 10.
The number 2 becomes the next digit in our result. 
Result so far: 2.0
Subtracting 42 from 52 leaves us with 10.
Fetch the next digit (5) from the dividend, attach it to 10 and continue: 105 / 21

Let's proceed to step 3:
21 can be fit into 105 5 times, resulting in a remainder of 0.
Use 5 as the next digit of our solution. 
Result so far: 25.0
If we subtract 105 from 105, we get 0.
Fetch the next digit (9) from the dividend, attach it to 0 and continue: 9 / 21

Going ahead to step 4:
When dividing 9 by 21, we get 0 with a remainder of 9.
Write down 0 as next digit of the result. 
Result so far: 250.0
The remainder is 9 after subtracting 0 from 9.
Fetch the next digit (8) from the dividend, attach it to 9 and continue: 98 / 21

Step 5:
98 divided by 21 is 4 with a remainder of 14.
The number 4 becomes the next digit in our result. 
Result so far: 2504.0
If we take 84 away from 98, we end up with 14.
Take the next digit (1) from the dividend and append it to 14, then repeat: 141 / 21

Moving on to step 6:
The number 21 fits into 141 6 times, leaving a remainder of 15.
Put 6 as the next digit of the answer. 
Result so far: 25046.0
If we subtract 126 from 141, we get 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Let's proceed to step 7:
The number 21 fits into 150 7 times, leaving a remainder of 3.
Write down 7 as next digit of the result. 
Result so far: 25046.7
The remainder is 3 after subtracting 147 from 150.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Step 8:
The number 21 fits into 30 1 times, leaving a remainder of 9.
Record the quotient 1 as the next digit in the result. 
Result so far: 25046.71
Subtracting 21 from 30 leaves us 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.

Step 9:
When dividing 90 by 21, we get 4 with a remainder of 6.
The number 4 becomes the next digit in our result. 
Result so far: 25046.714
Subtract 84 from 90 to get 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Moving on to step 10:
When dividing 60 by 21, we get 2 with a remainder of 18.
The next digit of our result is 2. 
Result so far: 25046.7142
Subtracting 42 from 60 leaves us 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.

Moving on to step 11:
The number 21 fits into 180 8 times, leaving a remainder of 12.
The next digit of our result is 8. 
Result so far: 25046.71428
Deduct 168 from 180 and we're left 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.

Let's proceed to step 12:
21 goes into 120 5 times with a remainder of 15.
Use 5 as the next digit of our solution. 
Result so far: 25046.714285
Subtract 105 from 120 to get 15.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Step 13:
21 goes into 150 7 times with a remainder of 3.
Write down 7 as next digit of the result. 
Result so far: 25046.7142857
If we take 147 away from 150, we end up with 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Step 14:
The number 21 fits into 30 1 times, leaving a remainder of 9.
The number 1 becomes the next digit in our result. 
Result so far: 25046.71428571
Subtracting 21 from 30 leaves us 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.

The final outcome is 25046.71428571, with a leftover of 9e-09.