We look at the division of 39960418 by 399 
The aim is to understand the frequency of 399 in 39960418.

Let's proceed to step 1:
The number 399 fits into 3 0 times, leaving a remainder of 3.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Subtracting 0 from 3 leaves us with 3.
Bring next digit (9) of the dividend behind the 3 and repeat the process: 39 / 399

Moving on to step 2:
The number 399 fits into 39 0 times, leaving a remainder of 39.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we take 0 away from 39, we end up with 39.
Append the next digit (9) from the dividend to 39 and continue with: 399 / 399

On to step 3:
399 goes into 399 1 times with a remainder of 0.
Record the quotient 1 as the next digit in the result. 
Result so far: 1.0
Subtracting 399 from 399 leaves us with 0.
Include the next digit (6) from the dividend after 0, then repeat: 6 / 399

Let's proceed to step 4:
When dividing 6 by 399, we get 0 with a remainder of 6.
Use 0 as the next digit of our solution. 
Result so far: 10.0
The remainder is 6 after subtracting 0 from 6.
Append the next digit (0) from the dividend to 6 and continue with: 60 / 399

Advancing to step 5:
399 goes into 60 0 times with a remainder of 60.
The next digit of our result is 0. 
Result so far: 100.0
Subtract 0 from 60 to get 60.
Bring next digit (4) of the dividend behind the 60 and repeat the process: 604 / 399

Step 6:
399 goes into 604 1 times with a remainder of 205.
Write down 1 as next digit of the result. 
Result so far: 1001.0
If we subtract 399 from 604, we get 205.
Fetch the next digit (1) from the dividend, attach it to 205 and continue: 2051 / 399

Advancing to step 7:
The number 399 fits into 2051 5 times, leaving a remainder of 56.
Record the quotient 5 as the next digit in the result. 
Result so far: 10015.0
Deduct 1995 from 2051 and we're left with 56.
Fetch the next digit (8) from the dividend, attach it to 56 and continue: 568 / 399

Let's proceed to step 8:
399 goes into 568 1 times with a remainder of 169.
The next digit of our result is 1. 
Result so far: 100151.0
If we take 399 away from 568, we end up with 169.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1690, and continue the process.

Advancing to step 9:
399 can be fit into 1690 4 times, resulting in a remainder of 94.
The number 4 becomes the next digit in our result. 
Result so far: 100151.4
Subtracting 1596 from 1690 leaves us with 94.
Since there are no more digits in the dividend, we add a zero to the remainder making it 940, and continue the process.

Let's proceed to step 10:
The number 399 fits into 940 2 times, leaving a remainder of 142.
Use 2 as the next digit of our solution. 
Result so far: 100151.42
Subtracting 798 from 940 leaves us with 142.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1420, and continue the process.

Moving on to step 11:
399 goes into 1420 3 times with a remainder of 223.
Write down 3 as next digit of the result. 
Result so far: 100151.423
Subtracting 1197 from 1420 leaves us with 223.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2230, and continue the process.

Advancing to step 12:
When dividing 2230 by 399, we get 5 with a remainder of 235.
The number 5 becomes the next digit in our result. 
Result so far: 100151.4235
If we take 1995 away from 2230, we end up with 235.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2350, and continue the process.

Moving on to step 13:
If we divide 2350 by 399, we get 5 and a remainder of 355.
The next digit of our result is 5. 
Result so far: 100151.42355
If we take 1995 away from 2350, we end up with 355.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3550, and continue the process.

The quotient of the division is 100151.42355, and the remainder is 0.000355.