We look at the division of 91151121 by 420 
The aim is to understand the frequency of 420 in 91151121.

On to step 1:
The number 420 fits into 9 0 times, leaving a remainder of 9.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we subtract 0 from 9, we get 9.
Grab the next digit (1) from the dividend, add it to 9, then carry on: 91 / 420

Advancing to step 2:
If we divide 91 by 420, we get 0 and a remainder of 91.
The next digit of our result is 0. 
Result so far: 0.0
Subtract 0 from 91 to get 91.
Append the next digit (1) from the dividend to 91 and continue with: 911 / 420

Moving on to step 3:
420 can be fit into 911 2 times, resulting in a remainder of 71.
Use 2 as the next digit of our solution. 
Result so far: 2.0
Subtracting 840 from 911 leaves us with 71.
Include the next digit (5) from the dividend after 71, then repeat: 715 / 420

Let's proceed to step 4:
If we divide 715 by 420, we get 1 and a remainder of 295.
The number 1 becomes the next digit in our result. 
Result so far: 21.0
Subtracting 420 from 715 leaves us with 295.
Fetch the next digit (1) from the dividend, attach it to 295 and continue: 2951 / 420

Step 5:
When dividing 2951 by 420, we get 7 with a remainder of 11.
Write down 7 as next digit of the result. 
Result so far: 217.0
Deduct 2940 from 2951 and we're left with 11.
Bring next digit (1) of the dividend behind the 11 and repeat the process: 111 / 420

On to step 6:
420 can be fit into 111 0 times, resulting in a remainder of 111.
The number 0 becomes the next digit in our result. 
Result so far: 2170.0
Subtract 0 from 111 to get 111.
Fetch the next digit (2) from the dividend, attach it to 111 and continue: 1112 / 420

Step 7:
1112 divided by 420 is 2 with a remainder of 272.
Use 2 as the next digit of our solution. 
Result so far: 21702.0
If we subtract 840 from 1112, we get 272.
Include the next digit (1) from the dividend after 272, then repeat: 2721 / 420

Going ahead to step 8:
420 can be fit into 2721 6 times, resulting in a remainder of 201.
Record the quotient 6 as the next digit in the result. 
Result so far: 217026.0
The remainder is 201 after subtracting 2520 from 2721.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2010, and continue the process.

Let's proceed to step 9:
The number 420 fits into 2010 4 times, leaving a remainder of 330.
Put 4 as the next digit of the answer. 
Result so far: 217026.4
If we subtract 1680 from 2010, we get 330.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3300, and continue the process.

Let's proceed to step 10:
When dividing 3300 by 420, we get 7 with a remainder of 360.
Record the quotient 7 as the next digit in the result. 
Result so far: 217026.47
If we take 2940 away from 3300, we end up with 360.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3600, and continue the process.

Step 11:
The number 420 fits into 3600 8 times, leaving a remainder of 240.
Write down 8 as next digit of the result. 
Result so far: 217026.478
Deduct 3360 from 3600 and we're left with 240.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2400, and continue the process.

Advancing to step 12:
The number 420 fits into 2400 5 times, leaving a remainder of 300.
The next digit of our result is 5. 
Result so far: 217026.4785
The remainder is 300 after subtracting 2100 from 2400.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3000, and continue the process.

Moving on to step 13:
When dividing 3000 by 420, we get 7 with a remainder of 60.
Record the quotient 7 as the next digit in the result. 
Result so far: 217026.47857
Deduct 2940 from 3000 and we're left with 60.
Since there are no more digits in the dividend, we add a zero to the remainder making it 600, and continue the process.

Advancing to step 14:
420 can be fit into 600 1 times, resulting in a remainder of 180.
The next digit of our result is 1. 
Result so far: 217026.478571
The remainder is 180 after subtracting 420 from 600.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1800, and continue the process.

Moving on to step 15:
1800 divided by 420 is 4 with a remainder of 120.
The next digit of our result is 4. 
Result so far: 217026.4785714
The remainder is 120 after subtracting 1680 from 1800.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1200, and continue the process.

Moving on to step 16:
When dividing 1200 by 420, we get 2 with a remainder of 360.
Record the quotient 2 as the next digit in the result. 
Result so far: 217026.47857142
Subtract 840 from 1200 to get 360.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3600, and continue the process.

Our division results in 217026.47857142 with a remaining 3.6e-07.