We look at the division of 210409 by 18 
The aim is to understand the frequency of 18 in 210409.

On to step 1:
2 divided by 18 is 0 with a remainder of 2.
The next digit of our result is 0. 
Result so far: 0.0
The remainder is 2 after subtracting 0 from 2.
Take the next digit (1) from the dividend and append it to 2, then repeat: 21 / 18

Moving on to step 2:
21 divided by 18 is 1 with a remainder of 3.
The number 1 becomes the next digit in our result. 
Result so far: 1.0
Subtract 18 from 21 to get 3.
Take the next digit (0) from the dividend and append it to 3, then repeat: 30 / 18

Step 3:
The number 18 fits into 30 1 times, leaving a remainder of 12.
The next digit of our result is 1. 
Result so far: 11.0
Subtract 18 from 30 to get 12.
Bring next digit (4) of the dividend behind the 12 and repeat the process: 124 / 18

Step 4:
The number 18 fits into 124 6 times, leaving a remainder of 16.
Use 6 as the next digit of our solution. 
Result so far: 116.0
If we subtract 108 from 124, we get 16.
Take the next digit (0) from the dividend and append it to 16, then repeat: 160 / 18

Step 5:
When dividing 160 by 18, we get 8 with a remainder of 16.
Record the quotient 8 as the next digit in the result. 
Result so far: 1168.0
If we subtract 144 from 160, we get 16.
Bring next digit (9) of the dividend behind the 16 and repeat the process: 169 / 18

Moving on to step 6:
18 goes into 169 9 times with a remainder of 7.
The next digit of our result is 9. 
Result so far: 11689.0
If we take 162 away from 169, 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.

Advancing to step 7:
If we divide 70 by 18, we get 3 and a remainder of 16.
Record the quotient 3 as the next digit in the result. 
Result so far: 11689.3
If we take 54 away from 70, we end up with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Step 8:
The number 18 fits into 160 8 times, leaving a remainder of 16.
Put 8 as the next digit of the answer. 
Result so far: 11689.38
Deduct 144 from 160 and we're left with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Let's proceed to step 9:
18 goes into 160 8 times with a remainder of 16.
Record the quotient 8 as the next digit in the result. 
Result so far: 11689.388
Deduct 144 from 160 and we're left with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

The final result is 11689.388 with a remainder of 0.0016.