We're dividing 21023561 by 399 
Let's see how many times 399 fits into 21023561.

Going ahead to step 1:
The number 399 fits into 2 0 times, leaving a remainder of 2.
Write down 0 as next digit of the result. 
Result so far: 0.0
Subtract 0 from 2 to get 2.
Grab the next digit (1) from the dividend, add it to 2, then carry on: 21 / 399

Moving on to step 2:
399 can be fit into 21 0 times, resulting in a remainder of 21.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Subtract 0 from 21 to get 21.
Include the next digit (0) from the dividend after 21, then repeat: 210 / 399

Moving on to step 3:
When dividing 210 by 399, we get 0 with a remainder of 210.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Subtracting 0 from 210 leaves us with 210.
Append the next digit (2) from the dividend to 210 and continue with: 2102 / 399

Advancing to step 4:
If we divide 2102 by 399, we get 5 and a remainder of 107.
The number 5 becomes the next digit in our result. 
Result so far: 5.0
Subtract 1995 from 2102 to get 107.
Append the next digit (3) from the dividend to 107 and continue with: 1073 / 399

Let's proceed to step 5:
The number 399 fits into 1073 2 times, leaving a remainder of 275.
The number 2 becomes the next digit in our result. 
Result so far: 52.0
Subtracting 798 from 1073 leaves us with 275.
Bring next digit (5) of the dividend behind the 275 and repeat the process: 2755 / 399

Going ahead to step 6:
399 goes into 2755 6 times with a remainder of 361.
Use 6 as the next digit of our solution. 
Result so far: 526.0
Subtracting 2394 from 2755 leaves us with 361.
Include the next digit (6) from the dividend after 361, then repeat: 3616 / 399

On to step 7:
399 can be fit into 3616 9 times, resulting in a remainder of 25.
Use 9 as the next digit of our solution. 
Result so far: 5269.0
The remainder is 25 after subtracting 3591 from 3616.
Grab the next digit (1) from the dividend, add it to 25, then carry on: 251 / 399

Step 8:
If we divide 251 by 399, we get 0 and a remainder of 251.
The next digit of our result is 0. 
Result so far: 52690.0
Subtracting 0 from 251 leaves us with 251.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2510, and continue the process.

Moving on to step 9:
When dividing 2510 by 399, we get 6 with a remainder of 116.
Put 6 as the next digit of the answer. 
Result so far: 52690.6
Subtracting 2394 from 2510 leaves us with 116.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1160, and continue the process.

Step 10:
399 can be fit into 1160 2 times, resulting in a remainder of 362.
Write down 2 as next digit of the result. 
Result so far: 52690.62
Subtract 798 from 1160 to get 362.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3620, and continue the process.

Moving on to step 11:
399 can be fit into 3620 9 times, resulting in a remainder of 29.
Put 9 as the next digit of the answer. 
Result so far: 52690.629
Subtract 3591 from 3620 to get 29.
Since there are no more digits in the dividend, we add a zero to the remainder making it 290, and continue the process.

Advancing to step 12:
When dividing 290 by 399, we get 0 with a remainder of 290.
The number 0 becomes the next digit in our result. 
Result so far: 52690.629
If we subtract 0 from 290, we get 290.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2900, and continue the process.

Let's proceed to step 13:
When dividing 2900 by 399, we get 7 with a remainder of 107.
The number 7 becomes the next digit in our result. 
Result so far: 52690.62907
Subtract 2793 from 2900 to get 107.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1070, and continue the process.

Moving on to step 14:
The number 399 fits into 1070 2 times, leaving a remainder of 272.
Put 2 as the next digit of the answer. 
Result so far: 52690.629072
If we subtract 798 from 1070, we get 272.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2720, and continue the process.

Step 15:
399 goes into 2720 6 times with a remainder of 326.
Put 6 as the next digit of the answer. 
Result so far: 52690.6290726
Subtracting 2394 from 2720 leaves us with 326.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3260, and continue the process.

Advancing to step 16:
399 can be fit into 3260 8 times, resulting in a remainder of 68.
Put 8 as the next digit of the answer. 
Result so far: 52690.62907268
Deduct 3192 from 3260 and we're left with 68.
Since there are no more digits in the dividend, we add a zero to the remainder making it 680, and continue the process.

After the division, we end up with 52690.62907268 and a remainder of 6.8e-08.