We look at the division of 66408988 by 521 
The aim is to understand the frequency of 521 in 66408988.

Moving on to step 1:
When dividing 6 by 521, we get 0 with a remainder of 6.
Put 0 as the next digit of the answer. 
Result so far: 0.0
Subtract 0 from 6 to get 6.
Fetch the next digit (6) from the dividend, attach it to 6 and continue: 66 / 521

On to step 2:
If we divide 66 by 521, we get 0 and a remainder of 66.
The next digit of our result is 0. 
Result so far: 0.0
The remainder is 66 after subtracting 0 from 66.
Append the next digit (4) from the dividend to 66 and continue with: 664 / 521

Advancing to step 3:
The number 521 fits into 664 1 times, leaving a remainder of 143.
The next digit of our result is 1. 
Result so far: 1.0
If we subtract 521 from 664, we get 143.
Bring next digit (0) of the dividend behind the 143 and repeat the process: 1430 / 521

Going ahead to step 4:
521 can be fit into 1430 2 times, resulting in a remainder of 388.
Record the quotient 2 as the next digit in the result. 
Result so far: 12.0
The remainder is 388 after subtracting 1042 from 1430.
Fetch the next digit (8) from the dividend, attach it to 388 and continue: 3888 / 521

Step 5:
3888 divided by 521 is 7 with a remainder of 241.
Record the quotient 7 as the next digit in the result. 
Result so far: 127.0
Subtract 3647 from 3888 to get 241.
Bring next digit (9) of the dividend behind the 241 and repeat the process: 2419 / 521

Going ahead to step 6:
521 goes into 2419 4 times with a remainder of 335.
The number 4 becomes the next digit in our result. 
Result so far: 1274.0
If we subtract 2084 from 2419, we get 335.
Include the next digit (8) from the dividend after 335, then repeat: 3358 / 521

Step 7:
If we divide 3358 by 521, we get 6 and a remainder of 232.
Put 6 as the next digit of the answer. 
Result so far: 12746.0
If we take 3126 away from 3358, we end up with 232.
Bring next digit (8) of the dividend behind the 232 and repeat the process: 2328 / 521

Moving on to step 8:
When dividing 2328 by 521, we get 4 with a remainder of 244.
Put 4 as the next digit of the answer. 
Result so far: 127464.0
Subtract 2084 from 2328 to get 244.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2440, and continue the process.

Going ahead to step 9:
2440 divided by 521 is 4 with a remainder of 356.
The number 4 becomes the next digit in our result. 
Result so far: 127464.4
Subtracting 2084 from 2440 leaves us with 356.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3560, and continue the process.

Going ahead to step 10:
521 can be fit into 3560 6 times, resulting in a remainder of 434.
Put 6 as the next digit of the answer. 
Result so far: 127464.46
If we subtract 3126 from 3560, we get 434.
Since there are no more digits in the dividend, we add a zero to the remainder making it 4340, and continue the process.

Going ahead to step 11:
When dividing 4340 by 521, we get 8 with a remainder of 172.
Record the quotient 8 as the next digit in the result. 
Result so far: 127464.468
If we subtract 4168 from 4340, we get 172.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1720, and continue the process.

Moving on to step 12:
When dividing 1720 by 521, we get 3 with a remainder of 157.
Put 3 as the next digit of the answer. 
Result so far: 127464.4683
The remainder is 157 after subtracting 1563 from 1720.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1570, and continue the process.

Let's proceed to step 13:
521 goes into 1570 3 times with a remainder of 7.
Record the quotient 3 as the next digit in the result. 
Result so far: 127464.46833
Subtract 1563 from 1570 to get 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Our division results in 127464.46833 with a remaining 7e-06.