We look at the division of 78921584 by 521 
The aim is to understand the frequency of 521 in 78921584.

Going ahead to step 1:
The number 521 fits into 7 0 times, leaving a remainder of 7.
Put 0 as the next digit of the answer. 
Result so far: 0.0
The remainder is 7 after subtracting 0 from 7.
Grab the next digit (8) from the dividend, add it to 7, then carry on: 78 / 521

On to step 2:
If we divide 78 by 521, we get 0 and a remainder of 78.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Deduct 0 from 78 and we're left with 78.
Grab the next digit (9) from the dividend, add it to 78, then carry on: 789 / 521

Going ahead to step 3:
789 divided by 521 is 1 with a remainder of 268.
Record the quotient 1 as the next digit in the result. 
Result so far: 1.0
If we take 521 away from 789, we end up with 268.
Grab the next digit (2) from the dividend, add it to 268, then carry on: 2682 / 521

Going ahead to step 4:
The number 521 fits into 2682 5 times, leaving a remainder of 77.
Record the quotient 5 as the next digit in the result. 
Result so far: 15.0
If we take 2605 away from 2682, we end up with 77.
Bring next digit (1) of the dividend behind the 77 and repeat the process: 771 / 521

Advancing to step 5:
521 goes into 771 1 times with a remainder of 250.
Write down 1 as next digit of the result. 
Result so far: 151.0
Subtract 521 from 771 to get 250.
Include the next digit (5) from the dividend after 250, then repeat: 2505 / 521

Moving on to step 6:
The number 521 fits into 2505 4 times, leaving a remainder of 421.
The next digit of our result is 4. 
Result so far: 1514.0
The remainder is 421 after subtracting 2084 from 2505.
Fetch the next digit (8) from the dividend, attach it to 421 and continue: 4218 / 521

Moving on to step 7:
521 can be fit into 4218 8 times, resulting in a remainder of 50.
Use 8 as the next digit of our solution. 
Result so far: 15148.0
If we subtract 4168 from 4218, we get 50.
Bring next digit (4) of the dividend behind the 50 and repeat the process: 504 / 521

Going ahead to step 8:
504 divided by 521 is 0 with a remainder of 504.
The number 0 becomes the next digit in our result. 
Result so far: 151480.0
The remainder is 504 after subtracting 0 from 504.
Since there are no more digits in the dividend, we add a zero to the remainder making it 5040, and continue the process.

Going ahead to step 9:
521 goes into 5040 9 times with a remainder of 351.
Record the quotient 9 as the next digit in the result. 
Result so far: 151480.9
If we take 4689 away from 5040, we end up with 351.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3510, and continue the process.

Moving on to step 10:
If we divide 3510 by 521, we get 6 and a remainder of 384.
Record the quotient 6 as the next digit in the result. 
Result so far: 151480.96
Subtract 3126 from 3510 to get 384.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3840, and continue the process.

Let's proceed to step 11:
3840 divided by 521 is 7 with a remainder of 193.
Record the quotient 7 as the next digit in the result. 
Result so far: 151480.967
Subtract 3647 from 3840 to get 193.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1930, and continue the process.

Let's proceed to step 12:
521 can be fit into 1930 3 times, resulting in a remainder of 367.
Record the quotient 3 as the next digit in the result. 
Result so far: 151480.9673
Subtract 1563 from 1930 to get 367.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3670, and continue the process.

Step 13:
If we divide 3670 by 521, we get 7 and a remainder of 23.
Write down 7 as next digit of the result. 
Result so far: 151480.96737
Subtracting 3647 from 3670 leaves us with 23.
Since there are no more digits in the dividend, we add a zero to the remainder making it 230, and continue the process.

Advancing to step 14:
521 goes into 230 0 times with a remainder of 230.
Use 0 as the next digit of our solution. 
Result so far: 151480.96737
Subtracting 0 from 230 leaves us with 230.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2300, and continue the process.

Moving on to step 15:
2300 divided by 521 is 4 with a remainder of 216.
The number 4 becomes the next digit in our result. 
Result so far: 151480.9673704
The remainder is 216 after subtracting 2084 from 2300.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2160, and continue the process.

Let's proceed to step 16:
521 can be fit into 2160 4 times, resulting in a remainder of 76.
Put 4 as the next digit of the answer. 
Result so far: 151480.96737044
If we take 2084 away from 2160, we end up with 76.
Since there are no more digits in the dividend, we add a zero to the remainder making it 760, and continue the process.

After the division, we end up with 151480.96737044 and a remainder of 7.6e-08.