We look at the division of 12439578 by 645 
Let's see how many times 645 fits into 12439578.

Moving on to step 1:
The number 645 fits into 1 0 times, leaving a remainder of 1.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Deduct 0 from 1 and we're left with 1.
Include the next digit (2) from the dividend after 1, then repeat: 12 / 645

Moving on to step 2:
If we divide 12 by 645, we get 0 and a remainder of 12.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Subtracting 0 from 12 leaves us with 12.
Take the next digit (4) from the dividend and append it to 12, then repeat: 124 / 645

Advancing to step 3:
The number 645 fits into 124 0 times, leaving a remainder of 124.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
The remainder is 124 after subtracting 0 from 124.
Include the next digit (3) from the dividend after 124, then repeat: 1243 / 645

Moving on to step 4:
If we divide 1243 by 645, we get 1 and a remainder of 598.
The number 1 becomes the next digit in our result. 
Result so far: 1.0
Subtracting 645 from 1243 leaves us with 598.
Take the next digit (9) from the dividend and append it to 598, then repeat: 5989 / 645

Going ahead to step 5:
645 can be fit into 5989 9 times, resulting in a remainder of 184.
Record the quotient 9 as the next digit in the result. 
Result so far: 19.0
If we subtract 5805 from 5989, we get 184.
Bring next digit (5) of the dividend behind the 184 and repeat the process: 1845 / 645

Step 6:
645 goes into 1845 2 times with a remainder of 555.
Record the quotient 2 as the next digit in the result. 
Result so far: 192.0
The remainder is 555 after subtracting 1290 from 1845.
Include the next digit (7) from the dividend after 555, then repeat: 5557 / 645

Going ahead to step 7:
645 can be fit into 5557 8 times, resulting in a remainder of 397.
Put 8 as the next digit of the answer. 
Result so far: 1928.0
Deduct 5160 from 5557 and we're left with 397.
Take the next digit (8) from the dividend and append it to 397, then repeat: 3978 / 645

Moving on to step 8:
When dividing 3978 by 645, we get 6 with a remainder of 108.
Record the quotient 6 as the next digit in the result. 
Result so far: 19286.0
Subtract 3870 from 3978 to get 108.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1080, and continue the process.

Let's proceed to step 9:
The number 645 fits into 1080 1 times, leaving a remainder of 435.
The next digit of our result is 1. 
Result so far: 19286.1
If we subtract 645 from 1080, we get 435.
Since there are no more digits in the dividend, we add a zero to the remainder making it 4350, and continue the process.

On to step 10:
645 can be fit into 4350 6 times, resulting in a remainder of 480.
Record the quotient 6 as the next digit in the result. 
Result so far: 19286.16
The remainder is 480 after subtracting 3870 from 4350.
Since there are no more digits in the dividend, we add a zero to the remainder making it 4800, and continue the process.

Advancing to step 11:
4800 divided by 645 is 7 with a remainder of 285.
The next digit of our result is 7. 
Result so far: 19286.167
Subtracting 4515 from 4800 leaves us with 285.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2850, and continue the process.

Let's proceed to step 12:
When dividing 2850 by 645, we get 4 with a remainder of 270.
The next digit of our result is 4. 
Result so far: 19286.1674
Subtract 2580 from 2850 to get 270.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2700, and continue the process.

On to step 13:
2700 divided by 645 is 4 with a remainder of 120.
The next digit of our result is 4. 
Result so far: 19286.16744
If we take 2580 away from 2700, we end up with 120.
Since there are no more digits in the dividend, we add a zero to the remainder making it 1200, and continue the process.

The final outcome is 19286.16744, with a leftover of 0.00012.