We're dividing 152910 by 21 
The aim is to understand the frequency of 21 in 152910.

Step 1:
The number 21 fits into 1 0 times, leaving a remainder of 1.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Deduct 0 from 1 and we're left with 1.
Grab the next digit (5) from the dividend, add it to 1, then carry on: 15 / 21

On to step 2:
When dividing 15 by 21, we get 0 with a remainder of 15.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
The remainder is 15 after subtracting 0 from 15.
Take the next digit (2) from the dividend and append it to 15, then repeat: 152 / 21

Advancing to step 3:
152 divided by 21 is 7 with a remainder of 5.
Write down 7 as next digit of the result. 
Result so far: 7.0
If we take 147 away from 152, we end up with 5.
Append the next digit (9) from the dividend to 5 and continue with: 59 / 21

Let's proceed to step 4:
21 can be fit into 59 2 times, resulting in a remainder of 17.
Record the quotient 2 as the next digit in the result. 
Result so far: 72.0
Subtract 42 from 59 to get 17.
Take the next digit (1) from the dividend and append it to 17, then repeat: 171 / 21

Let's proceed to step 5:
If we divide 171 by 21, we get 8 and a remainder of 3.
Write down 8 as next digit of the result. 
Result so far: 728.0
Subtract 168 from 171 to get 3.
Append the next digit (0) from the dividend to 3 and continue with: 30 / 21

Moving on to step 6:
If we divide 30 by 21, we get 1 and a remainder of 9.
Put 1 as the next digit of the answer. 
Result so far: 7281.0
If we subtract 21 from 30, we get 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Let's proceed to step 7:
90 divided by 21 is 4 with a remainder of 6.
Record the quotient 4 as the next digit in the result. 
Result so far: 7281.4
Subtracting 84 from 90 leaves us with 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Step 8:
The number 21 fits into 60 2 times, leaving a remainder of 18.
Put 2 as the next digit of the answer. 
Result so far: 7281.42
If we subtract 42 from 60, we get 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 9:
The number 21 fits into 180 8 times, leaving a remainder of 12.
Record the quotient 8 as the next digit in the result. 
Result so far: 7281.428
Subtract 168 from 180 to get 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Step 10:
If we divide 120 by 21, we get 5 and a remainder of 15.
The number 5 becomes the next digit in our result. 
Result so far: 7281.4285
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.

Moving on to step 11:
If we divide 150 by 21, we get 7 and a remainder of 3.
The next digit of our result is 7. 
Result so far: 7281.42857
Subtract 147 from 150 to 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.

Moving on to step 12:
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: 7281.428571
If we subtract 21 from 30, we get 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

On to step 13:
When dividing 90 by 21, we get 4 with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 7281.4285714
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.

Going ahead to step 14:
If we divide 60 by 21, we get 2 and a remainder of 18.
Use 2 as the next digit of our solution. 
Result so far: 7281.42857142
Deduct 42 from 60 and we're left 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.

The final result is 7281.42857142 with a remainder of 1.8e-08.