We're dividing 121604 by 15 
The aim is to understand the frequency of 15 in 121604.

On to step 1:
When dividing 1 by 15, we get 0 with a remainder of 1.
The next digit of our result is 0. 
Result so far: 0.0
Subtract 0 from 1 to get 1.
Bring next digit (2) of the dividend behind the 1 and repeat the process: 12 / 15

Moving on to step 2:
12 divided by 15 is 0 with a remainder of 12.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Deduct 0 from 12 and we're left with 12.
Take the next digit (1) from the dividend and append it to 12, then repeat: 121 / 15

Going ahead to step 3:
15 goes into 121 8 times with a remainder of 1.
Use 8 as the next digit of our solution. 
Result so far: 8.0
If we subtract 120 from 121, we get 1.
Append the next digit (6) from the dividend to 1 and continue with: 16 / 15

Let's proceed to step 4:
When dividing 16 by 15, we get 1 with a remainder of 1.
The next digit of our result is 1. 
Result so far: 81.0
The remainder is 1 after subtracting 15 from 16.
Fetch the next digit (0) from the dividend, attach it to 1 and continue: 10 / 15

Moving on to step 5:
If we divide 10 by 15, we get 0 and a remainder of 10.
The next digit of our result is 0. 
Result so far: 810.0
Deduct 0 from 10 and we're left with 10.
Fetch the next digit (4) from the dividend, attach it to 10 and continue: 104 / 15

Let's proceed to step 6:
104 divided by 15 is 6 with a remainder of 14.
Use 6 as the next digit of our solution. 
Result so far: 8106.0
The remainder is 14 after subtracting 90 from 104.
Since there are no more digits in the dividend, we add a zero to the remainder making it 140, and continue the process.

Moving on to step 7:
15 can be fit into 140 9 times, resulting in a remainder of 5.
Record the quotient 9 as the next digit in the result. 
Result so far: 8106.9
The remainder is 5 after subtracting 135 from 140.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

On to step 8:
The number 15 fits into 50 3 times, leaving a remainder of 5.
Put 3 as the next digit of the answer. 
Result so far: 8106.93
If we subtract 45 from 50, we get 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Let's proceed to step 9:
If we divide 50 by 15, we get 3 and a remainder of 5.
The next digit of our result is 3. 
Result so far: 8106.933
Subtracting 45 from 50 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.

Our division results in 8106.933 with a remaining 0.0005.