510810 ÷ 21 
We want to figure out the number of times 510810 can be divided by 21.

Let's proceed to step 1:
If we divide 5 by 21, we get 0 and a remainder of 5.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we take 0 away from 5, we end up with 5.
Grab the next digit (1) from the dividend, add it to 5, then carry on: 51 / 21

On to step 2:
The number 21 fits into 51 2 times, leaving a remainder of 9.
Write down 2 as next digit of the result. 
Result so far: 2.0
If we take 42 away from 51, we end up with 9.
Include the next digit (0) from the dividend after 9, then repeat: 90 / 21

On to step 3:
The number 21 fits into 90 4 times, leaving a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 24.0
Deduct 84 from 90 and we're left with 6.
Include the next digit (8) from the dividend after 6, then repeat: 68 / 21

Let's proceed to step 4:
68 divided by 21 is 3 with a remainder of 5.
Use 3 as the next digit of our solution. 
Result so far: 243.0
Deduct 63 from 68 and we're left with 5.
Fetch the next digit (1) from the dividend, attach it to 5 and continue: 51 / 21

Moving on to step 5:
51 divided by 21 is 2 with a remainder of 9.
The next digit of our result is 2. 
Result so far: 2432.0
The remainder is 9 after subtracting 42 from 51.
Append the next digit (0) from the dividend to 9 and continue with: 90 / 21

Moving on to step 6:
21 goes into 90 4 times with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 24324.0
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.

Let's proceed to step 7:
21 goes into 60 2 times with a remainder of 18.
Use 2 as the next digit of our solution. 
Result so far: 24324.2
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.

On to step 8:
21 can be fit into 180 8 times, resulting in a remainder of 12.
The next digit of our result is 8. 
Result so far: 24324.28
If we subtract 168 from 180, we get 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

Step 9:
The number 21 fits into 120 5 times, leaving a remainder of 15.
The next digit of our result is 5. 
Result so far: 24324.285
If we subtract 105 from 120, we 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.

Step 10:
21 can be fit into 150 7 times, resulting in a remainder of 3.
Put 7 as the next digit of the answer. 
Result so far: 24324.2857
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.

Let's proceed to step 11:
21 goes into 30 1 times with a remainder of 9.
Write down 1 as next digit of the result. 
Result so far: 24324.28571
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 12:
When dividing 90 by 21, we get 4 with a remainder of 6.
Put 4 as the next digit of the answer. 
Result so far: 24324.285714
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.

On to step 13:
60 divided by 21 is 2 with a remainder of 18.
Record the quotient 2 as the next digit in the result. 
Result so far: 24324.2857142
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 14:
When dividing 180 by 21, we get 8 with a remainder of 12.
Use 8 as the next digit of our solution. 
Result so far: 24324.28571428
If we subtract 168 from 180, we get 12.
Since there are no more digits in the dividend, we add a zero to the remainder making it 120, and continue the process.

The final result is 24324.28571428 with a remainder of 1.2e-08.