We look at the division of 645990 by 21 
We're looking to find how many times 21 goes into 645990.

Going ahead to step 1:
The number 21 fits into 6 0 times, leaving a remainder of 6.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we subtract 0 from 6, we get 6.
Include the next digit (4) from the dividend after 6, then repeat: 64 / 21

Going ahead to step 2:
The number 21 fits into 64 3 times, leaving a remainder of 1.
Put 3 as the next digit of the answer. 
Result so far: 3.0
The remainder is 1 after subtracting 63 from 64.
Take the next digit (5) from the dividend and append it to 1, then repeat: 15 / 21

Step 3:
The number 21 fits into 15 0 times, leaving a remainder of 15.
Write down 0 as next digit of the result. 
Result so far: 30.0
Subtract 0 from 15 to get 15.
Take the next digit (9) from the dividend and append it to 15, then repeat: 159 / 21

Going ahead to step 4:
159 divided by 21 is 7 with a remainder of 12.
The next digit of our result is 7. 
Result so far: 307.0
Subtract 147 from 159 to get 12.
Include the next digit (9) from the dividend after 12, then repeat: 129 / 21

Step 5:
If we divide 129 by 21, we get 6 and a remainder of 3.
Record the quotient 6 as the next digit in the result. 
Result so far: 3076.0
The remainder is 3 after subtracting 126 from 129.
Take the next digit (0) from the dividend and append it to 3, then repeat: 30 / 21

On to step 6:
When dividing 30 by 21, we get 1 with a remainder of 9.
Put 1 as the next digit of the answer. 
Result so far: 30761.0
Subtract 21 from 30 to 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.

Advancing to step 7:
If we divide 90 by 21, we get 4 and a remainder of 6.
Put 4 as the next digit of the answer. 
Result so far: 30761.4
The remainder is 6 after subtracting 84 from 90.
Since there are no more digits in the dividend, we add a zero to the remainder making it 60, and continue the process.

Advancing to step 8:
60 divided by 21 is 2 with a remainder of 18.
Write down 2 as next digit of the result. 
Result so far: 30761.42
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.

Step 9:
When dividing 180 by 21, we get 8 with a remainder of 12.
Put 8 as the next digit of the answer. 
Result so far: 30761.428
Deduct 168 from 180 and we're left with 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Moving on to step 10:
21 can be fit into 120 5 times, resulting in a remainder of 15.
Use 5 as the next digit of our solution. 
Result so far: 30761.4285
Subtract 105 from 120 to get 15.
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 11:
The number 21 fits into 150 7 times, leaving a remainder of 3.
Write down 7 as next digit of the result. 
Result so far: 30761.42857
Subtracting 147 from 150 leaves us with 3.
Since there are no more digits in the dividend, we add a zero to the remainder making it 30, and continue the process.

Going ahead to step 12:
30 divided by 21 is 1 with a remainder of 9.
The number 1 becomes the next digit in our result. 
Result so far: 30761.428571
The remainder is 9 after subtracting 21 from 30.
Since there are no more digits in the dividend, we add a zero to the remainder making it 90, and continue the process.

Step 13:
21 goes into 90 4 times with a remainder of 6.
Record the quotient 4 as the next digit in the result. 
Result so far: 30761.4285714
Subtracting 84 from 90 leaves us 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 14:
21 can be fit into 60 2 times, resulting in a remainder of 18.
Use 2 as the next digit of our solution. 
Result so far: 30761.42857142
If we take 42 away from 60, we end up 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 30761.42857142 with a remainder of 1.8e-08.