We're dividing 353660 by 21 
The aim is to understand the frequency of 21 in 353660.

Going ahead to step 1:
21 goes into 3 0 times with a remainder of 3.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Deduct 0 from 3 and we're left with 3.
Bring next digit (5) of the dividend behind the 3 and repeat the process: 35 / 21

Moving on to step 2:
21 can be fit into 35 1 times, resulting in a remainder of 14.
Write down 1 as next digit of the result. 
Result so far: 1.0
Deduct 21 from 35 and we're left with 14.
Grab the next digit (3) from the dividend, add it to 14, then carry on: 143 / 21

Advancing to step 3:
When dividing 143 by 21, we get 6 with a remainder of 17.
The number 6 becomes the next digit in our result. 
Result so far: 16.0
The remainder is 17 after subtracting 126 from 143.
Append the next digit (6) from the dividend to 17 and continue with: 176 / 21

Advancing to step 4:
When dividing 176 by 21, we get 8 with a remainder of 8.
Write down 8 as next digit of the result. 
Result so far: 168.0
If we subtract 168 from 176, we get 8.
Include the next digit (6) from the dividend after 8, then repeat: 86 / 21

Moving on to step 5:
86 divided by 21 is 4 with a remainder of 2.
Put 4 as the next digit of the answer. 
Result so far: 1684.0
Subtract 84 from 86 to get 2.
Take the next digit (0) from the dividend and append it to 2, then repeat: 20 / 21

On to step 6:
21 can be fit into 20 0 times, resulting in a remainder of 20.
Write down 0 as next digit of the result. 
Result so far: 16840.0
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.

Advancing to step 7:
The number 21 fits into 200 9 times, leaving a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 16840.9
If we subtract 189 from 200, we get 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Advancing to step 8:
21 can be fit into 110 5 times, resulting in a remainder of 5.
Use 5 as the next digit of our solution. 
Result so far: 16840.95
If we take 105 away from 110, we end up with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Advancing to step 9:
When dividing 50 by 21, we get 2 with a remainder of 8.
The next digit of our result is 2. 
Result so far: 16840.952
Deduct 42 from 50 and we're left with 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Let's proceed to step 10:
When dividing 80 by 21, we get 3 with a remainder of 17.
Put 3 as the next digit of the answer. 
Result so far: 16840.9523
The remainder is 17 after subtracting 63 from 80.
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 11:
The number 21 fits into 170 8 times, leaving a remainder of 2.
Put 8 as the next digit of the answer. 
Result so far: 16840.95238
Deduct 168 from 170 and we're left 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.

Moving on to step 12:
21 goes into 20 0 times with a remainder of 20.
The number 0 becomes the next digit in our result. 
Result so far: 16840.95238
If we subtract 0 from 20, we get 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

Going ahead to step 13:
21 goes into 200 9 times with a remainder of 11.
The number 9 becomes the next digit in our result. 
Result so far: 16840.9523809
The remainder is 11 after subtracting 189 from 200.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Step 14:
If we divide 110 by 21, we get 5 and a remainder of 5.
The number 5 becomes the next digit in our result. 
Result so far: 16840.95238095
Subtracting 105 from 110 leaves us with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

In conclusion, the division gives 16840.95238095 with a balance of 5e-09.