We look at the division of 535496 by 21 
The aim is to understand the frequency of 21 in 535496.

Advancing to step 1:
When dividing 5 by 21, we get 0 with a remainder of 5.
The next digit of our result is 0. 
Result so far: 0.0
The remainder is 5 after subtracting 0 from 5.
Include the next digit (3) from the dividend after 5, then repeat: 53 / 21

Going ahead to step 2:
The number 21 fits into 53 2 times, leaving a remainder of 11.
Put 2 as the next digit of the answer. 
Result so far: 2.0
Subtracting 42 from 53 leaves us with 11.
Bring next digit (5) of the dividend behind the 11 and repeat the process: 115 / 21

Moving on to step 3:
115 divided by 21 is 5 with a remainder of 10.
Put 5 as the next digit of the answer. 
Result so far: 25.0
Subtract 105 from 115 to get 10.
Fetch the next digit (4) from the dividend, attach it to 10 and continue: 104 / 21

On to step 4:
21 can be fit into 104 4 times, resulting in a remainder of 20.
The number 4 becomes the next digit in our result. 
Result so far: 254.0
Subtract 84 from 104 to get 20.
Grab the next digit (9) from the dividend, add it to 20, then carry on: 209 / 21

Going ahead to step 5:
If we divide 209 by 21, we get 9 and a remainder of 20.
Put 9 as the next digit of the answer. 
Result so far: 2549.0
If we subtract 189 from 209, we get 20.
Fetch the next digit (6) from the dividend, attach it to 20 and continue: 206 / 21

Advancing to step 6:
When dividing 206 by 21, we get 9 with a remainder of 17.
Write down 9 as next digit of the result. 
Result so far: 25499.0
If we take 189 away from 206, 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.

Going ahead to step 7:
170 divided by 21 is 8 with a remainder of 2.
The number 8 becomes the next digit in our result. 
Result so far: 25499.8
Subtracting 168 from 170 leaves us 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.

Let's proceed to step 8:
21 can be fit into 20 0 times, resulting in a remainder of 20.
Use 0 as the next digit of our solution. 
Result so far: 25499.8
Deduct 0 from 20 and we're left 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.

Let's proceed to step 9:
The number 21 fits into 200 9 times, leaving a remainder of 11.
Record the quotient 9 as the next digit in the result. 
Result so far: 25499.809
Subtracting 189 from 200 leaves us with 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 10:
When dividing 110 by 21, we get 5 with a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 25499.8095
If we subtract 105 from 110, we 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.

Moving on to step 11:
21 can be fit into 50 2 times, resulting in a remainder of 8.
Use 2 as the next digit of our solution. 
Result so far: 25499.80952
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.

Step 12:
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: 25499.809523
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.

Going ahead to step 13:
If we divide 170 by 21, we get 8 and a remainder of 2.
The next digit of our result is 8. 
Result so far: 25499.8095238
If we subtract 168 from 170, we get 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 14:
The number 21 fits into 20 0 times, leaving a remainder of 20.
The next digit of our result is 0. 
Result so far: 25499.8095238
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.

The quotient of the division is 25499.8095238, and the remainder is 2e-08.