We look at the division of 486129 by 21 
Let's see how many times 21 fits into 486129.

On to step 1:
21 can be fit into 4 0 times, resulting in a remainder of 4.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Deduct 0 from 4 and we're left with 4.
Include the next digit (8) from the dividend after 4, then repeat: 48 / 21

Let's proceed to step 2:
21 can be fit into 48 2 times, resulting in a remainder of 6.
The next digit of our result is 2. 
Result so far: 2.0
If we subtract 42 from 48, we get 6.
Include the next digit (6) from the dividend after 6, then repeat: 66 / 21

On to step 3:
21 goes into 66 3 times with a remainder of 3.
The number 3 becomes the next digit in our result. 
Result so far: 23.0
If we subtract 63 from 66, we get 3.
Take the next digit (1) from the dividend and append it to 3, then repeat: 31 / 21

Step 4:
When dividing 31 by 21, we get 1 with a remainder of 10.
The number 1 becomes the next digit in our result. 
Result so far: 231.0
If we take 21 away from 31, we end up with 10.
Bring next digit (2) of the dividend behind the 10 and repeat the process: 102 / 21

Step 5:
102 divided by 21 is 4 with a remainder of 18.
The next digit of our result is 4. 
Result so far: 2314.0
Deduct 84 from 102 and we're left with 18.
Take the next digit (9) from the dividend and append it to 18, then repeat: 189 / 21

Advancing to step 6:
If we divide 189 by 21, we get 9 and a remainder of 0.
The number 9 becomes the next digit in our result. 
Result so far: 23149.0
Subtract 189 from 189 to get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

On to step 7:
If we divide 0 by 21, we get 0 and a remainder of 0.
Put 0 as the next digit of the answer. 
Result so far: 23149.0
Subtracting 0 from 0 leaves us with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

On to step 8:
The number 21 fits into 0 0 times, leaving a remainder of 0.
The next digit of our result is 0. 
Result so far: 23149.0
Subtracting 0 from 0 leaves us with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Our division results in 23149.0 with a remaining 0.0.