We look at the division of 217125 by 21 
The aim is to understand the frequency of 21 in 217125.

Advancing to step 1:
When dividing 2 by 21, we get 0 with a remainder of 2.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
The remainder is 2 after subtracting 0 from 2.
Grab the next digit (1) from the dividend, add it to 2, then carry on: 21 / 21

Advancing to step 2:
If we divide 21 by 21, we get 1 and a remainder of 0.
The next digit of our result is 1. 
Result so far: 1.0
Deduct 21 from 21 and we're left with 0.
Bring next digit (7) of the dividend behind the 0 and repeat the process: 7 / 21

Going ahead to step 3:
When dividing 7 by 21, we get 0 with a remainder of 7.
The number 0 becomes the next digit in our result. 
Result so far: 10.0
Subtracting 0 from 7 leaves us with 7.
Grab the next digit (1) from the dividend, add it to 7, then carry on: 71 / 21

Going ahead to step 4:
When dividing 71 by 21, we get 3 with a remainder of 8.
Put 3 as the next digit of the answer. 
Result so far: 103.0
Subtracting 63 from 71 leaves us with 8.
Take the next digit (2) from the dividend and append it to 8, then repeat: 82 / 21

Going ahead to step 5:
If we divide 82 by 21, we get 3 and a remainder of 19.
Put 3 as the next digit of the answer. 
Result so far: 1033.0
If we subtract 63 from 82, we get 19.
Include the next digit (5) from the dividend after 19, then repeat: 195 / 21

Moving on to step 6:
21 goes into 195 9 times with a remainder of 6.
The next digit of our result is 9. 
Result so far: 10339.0
Subtract 189 from 195 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.

Let's proceed to step 7:
If we divide 60 by 21, we get 2 and a remainder of 18.
The next digit of our result is 2. 
Result so far: 10339.2
If we take 42 away from 60, we end up 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.

Advancing to step 8:
180 divided by 21 is 8 with a remainder of 12.
Record the quotient 8 as the next digit in the result. 
Result so far: 10339.28
The remainder is 12 after subtracting 168 from 180.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

On to step 9:
21 can be fit into 120 5 times, resulting in a remainder of 15.
Record the quotient 5 as the next digit in the result. 
Result so far: 10339.285
If we subtract 105 from 120, 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 10:
The number 21 fits into 150 7 times, leaving a remainder of 3.
Record the quotient 7 as the next digit in the result. 
Result so far: 10339.2857
If we subtract 147 from 150, we get 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Let's proceed to step 11:
When dividing 30 by 21, we get 1 with a remainder of 9.
Put 1 as the next digit of the answer. 
Result so far: 10339.28571
If we take 21 away from 30, we end up 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.

Moving on to step 12:
21 goes into 90 4 times with a remainder of 6.
Put 4 as the next digit of the answer. 
Result so far: 10339.285714
The remainder is 6 after subtracting 84 from 90.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Going ahead to step 13:
When dividing 60 by 21, we get 2 with a remainder of 18.
Put 2 as the next digit of the answer. 
Result so far: 10339.2857142
If we take 42 away from 60, we end up 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.

Step 14:
If we divide 180 by 21, we get 8 and a remainder of 12.
Put 8 as the next digit of the answer. 
Result so far: 10339.28571428
Subtracting 168 from 180 leaves us 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.

The final outcome is 10339.28571428, with a leftover of 1.2e-08.