We look at the division of 517258 by 21 
The aim is to understand the frequency of 21 in 517258.

Moving on to step 1:
The number 21 fits into 5 0 times, leaving a remainder of 5.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Deduct 0 from 5 and we're left with 5.
Fetch the next digit (1) from the dividend, attach it to 5 and continue: 51 / 21

Advancing to step 2:
51 divided by 21 is 2 with a remainder of 9.
The number 2 becomes the next digit in our result. 
Result so far: 2.0
Subtract 42 from 51 to get 9.
Include the next digit (7) from the dividend after 9, then repeat: 97 / 21

Let's proceed to step 3:
When dividing 97 by 21, we get 4 with a remainder of 13.
Put 4 as the next digit of the answer. 
Result so far: 24.0
If we take 84 away from 97, we end up with 13.
Include the next digit (2) from the dividend after 13, then repeat: 132 / 21

Advancing to step 4:
132 divided by 21 is 6 with a remainder of 6.
Put 6 as the next digit of the answer. 
Result so far: 246.0
The remainder is 6 after subtracting 126 from 132.
Grab the next digit (5) from the dividend, add it to 6, then carry on: 65 / 21

Step 5:
21 can be fit into 65 3 times, resulting in a remainder of 2.
Put 3 as the next digit of the answer. 
Result so far: 2463.0
If we take 63 away from 65, we end up with 2.
Include the next digit (8) from the dividend after 2, then repeat: 28 / 21

Step 6:
If we divide 28 by 21, we get 1 and a remainder of 7.
Write down 1 as next digit of the result. 
Result so far: 24631.0
If we take 21 away from 28, 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.

Let's proceed to step 7:
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: 24631.3
Subtracting 63 from 70 leaves us 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.

Advancing to step 8:
70 divided by 21 is 3 with a remainder of 7.
Write down 3 as next digit of the result. 
Result so far: 24631.33
The remainder is 7 after subtracting 63 from 70.
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 outcome is 24631.33, with a leftover of 0.007.