We look at the division of 66101543 by 399 
The aim is to understand the frequency of 399 in 66101543.

Let's proceed to step 1:
When dividing 6 by 399, we get 0 with a remainder of 6.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtract 0 from 6 to get 6.
Include the next digit (6) from the dividend after 6, then repeat: 66 / 399

Advancing to step 2:
399 can be fit into 66 0 times, resulting in a remainder of 66.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we subtract 0 from 66, we get 66.
Fetch the next digit (1) from the dividend, attach it to 66 and continue: 661 / 399

Advancing to step 3:
If we divide 661 by 399, we get 1 and a remainder of 262.
Put 1 as the next digit of the answer. 
Result so far: 1.0
Subtracting 399 from 661 leaves us with 262.
Append the next digit (0) from the dividend to 262 and continue with: 2620 / 399

Moving on to step 4:
399 can be fit into 2620 6 times, resulting in a remainder of 226.
Use 6 as the next digit of our solution. 
Result so far: 16.0
If we take 2394 away from 2620, we end up with 226.
Include the next digit (1) from the dividend after 226, then repeat: 2261 / 399

Advancing to step 5:
399 goes into 2261 5 times with a remainder of 266.
Put 5 as the next digit of the answer. 
Result so far: 165.0
Deduct 1995 from 2261 and we're left with 266.
Bring next digit (5) of the dividend behind the 266 and repeat the process: 2665 / 399

Step 6:
399 can be fit into 2665 6 times, resulting in a remainder of 271.
Put 6 as the next digit of the answer. 
Result so far: 1656.0
If we subtract 2394 from 2665, we get 271.
Include the next digit (4) from the dividend after 271, then repeat: 2714 / 399

Step 7:
If we divide 2714 by 399, we get 6 and a remainder of 320.
Record the quotient 6 as the next digit in the result. 
Result so far: 16566.0
Subtracting 2394 from 2714 leaves us with 320.
Bring next digit (3) of the dividend behind the 320 and repeat the process: 3203 / 399

Going ahead to step 8:
3203 divided by 399 is 8 with a remainder of 11.
The number 8 becomes the next digit in our result. 
Result so far: 165668.0
If we subtract 3192 from 3203, we get 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

On to step 9:
When dividing 110 by 399, we get 0 with a remainder of 110.
Use 0 as the next digit of our solution. 
Result so far: 165668.0
Subtracting 0 from 110 leaves us with 110.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1100, and continue the process.

Step 10:
399 can be fit into 1100 2 times, resulting in a remainder of 302.
The number 2 becomes the next digit in our result. 
Result so far: 165668.02
The remainder is 302 after subtracting 798 from 1100.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3020, and continue the process.

Step 11:
If we divide 3020 by 399, we get 7 and a remainder of 227.
The next digit of our result is 7. 
Result so far: 165668.027
If we subtract 2793 from 3020, we get 227.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2270, and continue the process.

Let's proceed to step 12:
2270 divided by 399 is 5 with a remainder of 275.
The next digit of our result is 5. 
Result so far: 165668.0275
If we take 1995 away from 2270, we end up with 275.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2750, and continue the process.

Advancing to step 13:
If we divide 2750 by 399, we get 6 and a remainder of 356.
Write down 6 as next digit of the result. 
Result so far: 165668.02756
If we subtract 2394 from 2750, we get 356.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3560, and continue the process.

Advancing to step 14:
3560 divided by 399 is 8 with a remainder of 368.
Record the quotient 8 as the next digit in the result. 
Result so far: 165668.027568
Subtract 3192 from 3560 to get 368.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3680, and continue the process.

Let's proceed to step 15:
3680 divided by 399 is 9 with a remainder of 89.
Use 9 as the next digit of our solution. 
Result so far: 165668.0275689
The remainder is 89 after subtracting 3591 from 3680.
Since there are no more digits in the dividend, we add a zero to the remainder making it 890, and continue the process.

Step 16:
The number 399 fits into 890 2 times, leaving a remainder of 92.
Record the quotient 2 as the next digit in the result. 
Result so far: 165668.02756892
Subtract 798 from 890 to get 92.
Since there are no more digits in the dividend, we add a zero to the remainder making it 920, and continue the process.

The quotient of the division is 165668.02756892, and the remainder is 9.2e-08.