We look at the division of 398437 by 91 
The aim is to understand the frequency of 91 in 398437.

Going ahead to step 1:
3 divided by 91 is 0 with a remainder of 3.
Put 0 as the next digit of the answer. 
Result so far: 0.0
The remainder is 3 after subtracting 0 from 3.
Bring next digit (9) of the dividend behind the 3 and repeat the process: 39 / 91

On to step 2:
91 can be fit into 39 0 times, resulting in a remainder of 39.
Put 0 as the next digit of the answer. 
Result so far: 0.0
Subtract 0 from 39 to get 39.
Include the next digit (8) from the dividend after 39, then repeat: 398 / 91

Step 3:
398 divided by 91 is 4 with a remainder of 34.
The number 4 becomes the next digit in our result. 
Result so far: 4.0
If we take 364 away from 398, we end up with 34.
Include the next digit (4) from the dividend after 34, then repeat: 344 / 91

Going ahead to step 4:
344 divided by 91 is 3 with a remainder of 71.
Put 3 as the next digit of the answer. 
Result so far: 43.0
The remainder is 71 after subtracting 273 from 344.
Bring next digit (3) of the dividend behind the 71 and repeat the process: 713 / 91

Step 5:
713 divided by 91 is 7 with a remainder of 76.
Put 7 as the next digit of the answer. 
Result so far: 437.0
If we subtract 637 from 713, we get 76.
Include the next digit (7) from the dividend after 76, then repeat: 767 / 91

Advancing to step 6:
When dividing 767 by 91, we get 8 with a remainder of 39.
Put 8 as the next digit of the answer. 
Result so far: 4378.0
The remainder is 39 after subtracting 728 from 767.
Since there are no more digits in the dividend, we add a zero to the remainder making it 390, and continue the process.

Moving on to step 7:
91 goes into 390 4 times with a remainder of 26.
Put 4 as the next digit of the answer. 
Result so far: 4378.4
Subtracting 364 from 390 leaves us with 26.
Since there are no more digits in the dividend, we add a zero to the remainder making it 260, and continue the process.

Let's proceed to step 8:
91 can be fit into 260 2 times, resulting in a remainder of 78.
Put 2 as the next digit of the answer. 
Result so far: 4378.42
Deduct 182 from 260 and we're left with 78.
Since there are no more digits in the dividend, we add a zero to the remainder making it 780, and continue the process.

On to step 9:
91 goes into 780 8 times with a remainder of 52.
Record the quotient 8 as the next digit in the result. 
Result so far: 4378.428
If we subtract 728 from 780, we get 52.
Since there are no more digits in the dividend, we add a zero to the remainder making it 520, and continue the process.

On to step 10:
The number 91 fits into 520 5 times, leaving a remainder of 65.
Put 5 as the next digit of the answer. 
Result so far: 4378.4285
Subtracting 455 from 520 leaves us with 65.
Since there are no more digits in the dividend, we add a zero to the remainder making it 650, and continue the process.

Moving on to step 11:
When dividing 650 by 91, we get 7 with a remainder of 13.
The next digit of our result is 7. 
Result so far: 4378.42857
If we subtract 637 from 650, we get 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Moving on to step 12:
91 goes into 130 1 times with a remainder of 39.
The number 1 becomes the next digit in our result. 
Result so far: 4378.428571
The remainder is 39 after subtracting 91 from 130.
Since there are no more digits in the dividend, we add a zero to the remainder making it 390, and continue the process.

Step 13:
91 goes into 390 4 times with a remainder of 26.
Record the quotient 4 as the next digit in the result. 
Result so far: 4378.4285714
The remainder is 26 after subtracting 364 from 390.
Since there are no more digits in the dividend, we add a zero to the remainder making it 260, and continue the process.

Moving on to step 14:
The number 91 fits into 260 2 times, leaving a remainder of 78.
The number 2 becomes the next digit in our result. 
Result so far: 4378.42857142
The remainder is 78 after subtracting 182 from 260.
Since there are no more digits in the dividend, we add a zero to the remainder making it 780, and continue the process.

Our division results in 4378.42857142 with a remaining 7.8e-08.