We divide 12399 by 21
Our goal is to divide 12399 by 21.

Going ahead to step 1:
21 goes into 1 0 times with a remainder of 1.
The next digit of our result is 0. 
Result so far: 0.0
If we take 0 away from 1, we end up with 1.
Take the next digit (2) from the dividend and append it to 1, then repeat: 12 / 21

Going ahead to step 2:
If we divide 12 by 21, we get 0 and a remainder of 12.
Put 0 as the next digit of the answer. 
Result so far: 0.0
If we subtract 0 from 12, we get 12.
Include the next digit (3) from the dividend after 12, then repeat: 123 / 21

Step 3:
If we divide 123 by 21, we get 5 and a remainder of 18.
Write down 5 as next digit of the result. 
Result so far: 5.0
If we take 105 away from 123, we end up with 18.
Append the next digit (9) from the dividend to 18 and continue with: 189 / 21

Step 4:
21 goes into 189 9 times with a remainder of 0.
The next digit of our result is 9. 
Result so far: 59.0
Subtracting 189 from 189 leaves us with 0.
Fetch the next digit (9) from the dividend, attach it to 0 and continue: 9 / 21

Step 5:
21 goes into 9 0 times with a remainder of 9.
The next digit of our result is 0. 
Result so far: 590.0
Subtracting 0 from 9 leaves us with 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Let's proceed to step 6:
When dividing 90 by 21, we get 4 with a remainder of 6.
Use 4 as the next digit of our solution. 
Result so far: 590.4
If we subtract 84 from 90, we get 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Going ahead to step 7:
The number 21 fits into 60 2 times, leaving a remainder of 18.
Put 2 as the next digit of the answer. 
Result so far: 590.42
Subtract 42 from 60 to get 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

Moving on to step 8:
The number 21 fits into 180 8 times, leaving a remainder of 12.
Record the quotient 8 as the next digit in the result. 
Result so far: 590.428
The remainder is 12 after subtracting 168 from 180.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Let's proceed to step 9:
When dividing 120 by 21, we get 5 with a remainder of 15.
Use 5 as the next digit of our solution. 
Result so far: 590.4285
The remainder is 15 after subtracting 105 from 120.
Since there are no more digits in the dividend, we add a zero to the remainder making it 150, and continue the process.

Going ahead to step 10:
If we divide 150 by 21, we get 7 and a remainder of 3.
The number 7 becomes the next digit in our result. 
Result so far: 590.42857
The remainder is 3 after subtracting 147 from 150.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Advancing to step 11:
The number 21 fits into 30 1 times, leaving a remainder of 9.
Use 1 as the next digit of our solution. 
Result so far: 590.428571
If we subtract 21 from 30, we get 9.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Step 12:
The number 21 fits into 90 4 times, leaving a remainder of 6.
The next digit of our result is 4. 
Result so far: 590.4285714
If we take 84 away from 90, we end up with 6.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Moving on to step 13:
The number 21 fits into 60 2 times, leaving a remainder of 18.
The next digit of our result is 2. 
Result so far: 590.42857142
Deduct 42 from 60 and we're left with 18.
Since there are no more digits in the dividend, we add a zero to the remainder making it 180, and continue the process.

The final result is 590.42857142 with a remainder of 1.8e-08.