51221827 ÷ 399 
The aim is to understand the frequency of 399 in 51221827.

Moving on to step 1:
399 can be fit into 5 0 times, resulting in a remainder of 5.
The next digit of our result is 0. 
Result so far: 0.0
The remainder is 5 after subtracting 0 from 5.
Bring next digit (1) of the dividend behind the 5 and repeat the process: 51 / 399

Going ahead to step 2:
If we divide 51 by 399, we get 0 and a remainder of 51.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Deduct 0 from 51 and we're left with 51.
Bring next digit (2) of the dividend behind the 51 and repeat the process: 512 / 399

Going ahead to step 3:
If we divide 512 by 399, we get 1 and a remainder of 113.
Use 1 as the next digit of our solution. 
Result so far: 1.0
Deduct 399 from 512 and we're left with 113.
Take the next digit (2) from the dividend and append it to 113, then repeat: 1132 / 399

Advancing to step 4:
1132 divided by 399 is 2 with a remainder of 334.
Put 2 as the next digit of the answer. 
Result so far: 12.0
Subtract 798 from 1132 to get 334.
Include the next digit (1) from the dividend after 334, then repeat: 3341 / 399

Going ahead to step 5:
The number 399 fits into 3341 8 times, leaving a remainder of 149.
The next digit of our result is 8. 
Result so far: 128.0
If we subtract 3192 from 3341, we get 149.
Fetch the next digit (8) from the dividend, attach it to 149 and continue: 1498 / 399

On to step 6:
The number 399 fits into 1498 3 times, leaving a remainder of 301.
Record the quotient 3 as the next digit in the result. 
Result so far: 1283.0
If we take 1197 away from 1498, we end up with 301.
Grab the next digit (2) from the dividend, add it to 301, then carry on: 3012 / 399

Moving on to step 7:
399 can be fit into 3012 7 times, resulting in a remainder of 219.
The number 7 becomes the next digit in our result. 
Result so far: 12837.0
If we subtract 2793 from 3012, we get 219.
Fetch the next digit (7) from the dividend, attach it to 219 and continue: 2197 / 399

On to step 8:
399 goes into 2197 5 times with a remainder of 202.
Record the quotient 5 as the next digit in the result. 
Result so far: 128375.0
If we subtract 1995 from 2197, we get 202.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2020, and continue the process.

On to step 9:
2020 divided by 399 is 5 with a remainder of 25.
Put 5 as the next digit of the answer. 
Result so far: 128375.5
The remainder is 25 after subtracting 1995 from 2020.
Since there are no more digits in the dividend, we add a zero to the remainder making it 250, and continue the process.

Step 10:
The number 399 fits into 250 0 times, leaving a remainder of 250.
Put 0 as the next digit of the answer. 
Result so far: 128375.5
Subtract 0 from 250 to get 250.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2500, and continue the process.

Moving on to step 11:
399 can be fit into 2500 6 times, resulting in a remainder of 106.
The number 6 becomes the next digit in our result. 
Result so far: 128375.506
The remainder is 106 after subtracting 2394 from 2500.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1060, and continue the process.

Going ahead to step 12:
1060 divided by 399 is 2 with a remainder of 262.
Record the quotient 2 as the next digit in the result. 
Result so far: 128375.5062
Subtracting 798 from 1060 leaves us with 262.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2620, and continue the process.

On to step 13:
399 goes into 2620 6 times with a remainder of 226.
Record the quotient 6 as the next digit in the result. 
Result so far: 128375.50626
Subtracting 2394 from 2620 leaves us with 226.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2260, and continue the process.

Going ahead to step 14:
399 goes into 2260 5 times with a remainder of 265.
Use 5 as the next digit of our solution. 
Result so far: 128375.506265
Subtracting 1995 from 2260 leaves us with 265.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2650, and continue the process.

Advancing to step 15:
If we divide 2650 by 399, we get 6 and a remainder of 256.
Use 6 as the next digit of our solution. 
Result so far: 128375.5062656
If we take 2394 away from 2650, we end up with 256.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2560, and continue the process.

Let's proceed to step 16:
399 can be fit into 2560 6 times, resulting in a remainder of 166.
Use 6 as the next digit of our solution. 
Result so far: 128375.50626566
Subtract 2394 from 2560 to get 166.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1660, and continue the process.

Our division results in 128375.50626566 with a remaining 1.66e-07.