We divide 1557409 by 399
The aim is to understand the frequency of 399 in 1557409.

Advancing to step 1:
1 divided by 399 is 0 with a remainder of 1.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we subtract 0 from 1, we get 1.
Grab the next digit (5) from the dividend, add it to 1, then carry on: 15 / 399

Let's proceed to step 2:
15 divided by 399 is 0 with a remainder of 15.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we subtract 0 from 15, we get 15.
Bring next digit (5) of the dividend behind the 15 and repeat the process: 155 / 399

Moving on to step 3:
If we divide 155 by 399, we get 0 and a remainder of 155.
Write down 0 as next digit of the result. 
Result so far: 0.0
If we take 0 away from 155, we end up with 155.
Append the next digit (7) from the dividend to 155 and continue with: 1557 / 399

On to step 4:
When dividing 1557 by 399, we get 3 with a remainder of 360.
Write down 3 as next digit of the result. 
Result so far: 3.0
If we subtract 1197 from 1557, we get 360.
Fetch the next digit (4) from the dividend, attach it to 360 and continue: 3604 / 399

Going ahead to step 5:
The number 399 fits into 3604 9 times, leaving a remainder of 13.
Record the quotient 9 as the next digit in the result. 
Result so far: 39.0
Subtract 3591 from 3604 to get 13.
Take the next digit (0) from the dividend and append it to 13, then repeat: 130 / 399

Let's proceed to step 6:
130 divided by 399 is 0 with a remainder of 130.
Record the quotient 0 as the next digit in the result. 
Result so far: 390.0
Subtract 0 from 130 to get 130.
Take the next digit (9) from the dividend and append it to 130, then repeat: 1309 / 399

Step 7:
When dividing 1309 by 399, we get 3 with a remainder of 112.
Write down 3 as next digit of the result. 
Result so far: 3903.0
The remainder is 112 after subtracting 1197 from 1309.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1120, and continue the process.

Moving on to step 8:
The number 399 fits into 1120 2 times, leaving a remainder of 322.
Put 2 as the next digit of the answer. 
Result so far: 3903.2
If we take 798 away from 1120, we end up with 322.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3220, and continue the process.

Moving on to step 9:
3220 divided by 399 is 8 with a remainder of 28.
Put 8 as the next digit of the answer. 
Result so far: 3903.28
The remainder is 28 after subtracting 3192 from 3220.
Since there are no more digits in the dividend, we add a zero to the remainder making it 280, and continue the process.

Step 10:
399 can be fit into 280 0 times, resulting in a remainder of 280.
The number 0 becomes the next digit in our result. 
Result so far: 3903.28
Subtracting 0 from 280 leaves us with 280.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2800, and continue the process.

Going ahead to step 11:
The number 399 fits into 2800 7 times, leaving a remainder of 7.
Record the quotient 7 as the next digit in the result. 
Result so far: 3903.2807
Deduct 2793 from 2800 and we're left with 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

On to step 12:
When dividing 70 by 399, we get 0 with a remainder of 70.
Record the quotient 0 as the next digit in the result. 
Result so far: 3903.2807
Subtracting 0 from 70 leaves us with 70.
Since there are no more digits in the dividend, we add a zero to the remainder making it 700, and continue the process.

Moving on to step 13:
The number 399 fits into 700 1 times, leaving a remainder of 301.
The next digit of our result is 1. 
Result so far: 3903.280701
If we subtract 399 from 700, we get 301.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3010, and continue the process.

Let's proceed to step 14:
399 can be fit into 3010 7 times, resulting in a remainder of 217.
Put 7 as the next digit of the answer. 
Result so far: 3903.2807017
Deduct 2793 from 3010 and we're left with 217.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2170, and continue the process.

Going ahead to step 15:
The number 399 fits into 2170 5 times, leaving a remainder of 175.
The next digit of our result is 5. 
Result so far: 3903.28070175
If we take 1995 away from 2170, we end up with 175.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1750, and continue the process.

After the division, we end up with 3903.28070175 and a remainder of 1.75e-07.