We divide 661863 by 90
The aim is to understand the frequency of 90 in 661863.

Going ahead to step 1:
If we divide 6 by 90, we get 0 and a remainder of 6.
Write down 0 as next digit of the result. 
Result so far: 0.0
Subtracting 0 from 6 leaves us with 6.
Grab the next digit (6) from the dividend, add it to 6, then carry on: 66 / 90

Moving on to step 2:
90 goes into 66 0 times with a remainder of 66.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we take 0 away from 66, we end up with 66.
Include the next digit (1) from the dividend after 66, then repeat: 661 / 90

Moving on to step 3:
90 goes into 661 7 times with a remainder of 31.
Put 7 as the next digit of the answer. 
Result so far: 7.0
Subtracting 630 from 661 leaves us with 31.
Include the next digit (8) from the dividend after 31, then repeat: 318 / 90

Going ahead to step 4:
When dividing 318 by 90, we get 3 with a remainder of 48.
Record the quotient 3 as the next digit in the result. 
Result so far: 73.0
Subtracting 270 from 318 leaves us with 48.
Bring next digit (6) of the dividend behind the 48 and repeat the process: 486 / 90

Step 5:
486 divided by 90 is 5 with a remainder of 36.
Record the quotient 5 as the next digit in the result. 
Result so far: 735.0
If we take 450 away from 486, we end up with 36.
Bring next digit (3) of the dividend behind the 36 and repeat the process: 363 / 90

On to step 6:
If we divide 363 by 90, we get 4 and a remainder of 3.
Use 4 as the next digit of our solution. 
Result so far: 7354.0
Subtracting 360 from 363 leaves us with 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Let's proceed to step 7:
If we divide 30 by 90, we get 0 and a remainder of 30.
Use 0 as the next digit of our solution. 
Result so far: 7354.0
The remainder is 30 after subtracting 0 from 30.
Since there are no more digits in the dividend, we add a zero to the remainder making it 300, and continue the process.

Going ahead to step 8:
If we divide 300 by 90, we get 3 and a remainder of 30.
The next digit of our result is 3. 
Result so far: 7354.03
Subtract 270 from 300 to get 30.
Since there are no more digits in the dividend, we add a zero to the remainder making it 300, and continue the process.

Advancing to step 9:
90 can be fit into 300 3 times, resulting in a remainder of 30.
Put 3 as the next digit of the answer. 
Result so far: 7354.033
If we subtract 270 from 300, we get 30.
Since there are no more digits in the dividend, we add a zero to the remainder making it 300, and continue the process.

Our division results in 7354.033 with a remaining 0.003.