8511921 ÷ 645 
Let's see how many times 645 fits into 8511921.

Advancing to step 1:
If we divide 8 by 645, we get 0 and a remainder of 8.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Subtract 0 from 8 to get 8.
Fetch the next digit (5) from the dividend, attach it to 8 and continue: 85 / 645

Moving on to step 2:
85 divided by 645 is 0 with a remainder of 85.
The next digit of our result is 0. 
Result so far: 0.0
If we subtract 0 from 85, we get 85.
Fetch the next digit (1) from the dividend, attach it to 85 and continue: 851 / 645

Advancing to step 3:
The number 645 fits into 851 1 times, leaving a remainder of 206.
The next digit of our result is 1. 
Result so far: 1.0
If we take 645 away from 851, we end up with 206.
Bring next digit (1) of the dividend behind the 206 and repeat the process: 2061 / 645

On to step 4:
2061 divided by 645 is 3 with a remainder of 126.
Record the quotient 3 as the next digit in the result. 
Result so far: 13.0
The remainder is 126 after subtracting 1935 from 2061.
Bring next digit (9) of the dividend behind the 126 and repeat the process: 1269 / 645

Step 5:
If we divide 1269 by 645, we get 1 and a remainder of 624.
The number 1 becomes the next digit in our result. 
Result so far: 131.0
Subtracting 645 from 1269 leaves us with 624.
Grab the next digit (2) from the dividend, add it to 624, then carry on: 6242 / 645

Let's proceed to step 6:
645 goes into 6242 9 times with a remainder of 437.
Put 9 as the next digit of the answer. 
Result so far: 1319.0
If we take 5805 away from 6242, we end up with 437.
Grab the next digit (1) from the dividend, add it to 437, then carry on: 4371 / 645

Let's proceed to step 7:
When dividing 4371 by 645, we get 6 with a remainder of 501.
Write down 6 as next digit of the result. 
Result so far: 13196.0
Subtract 3870 from 4371 to get 501.
Since there are no more digits in the dividend, we add a zero to the remainder making it 5010, and continue the process.

Moving on to step 8:
If we divide 5010 by 645, we get 7 and a remainder of 495.
The next digit of our result is 7. 
Result so far: 13196.7
If we subtract 4515 from 5010, we get 495.
Since there are no more digits in the dividend, we add a zero to the remainder making it 4950, and continue the process.

Moving on to step 9:
The number 645 fits into 4950 7 times, leaving a remainder of 435.
Put 7 as the next digit of the answer. 
Result so far: 13196.77
Subtract 4515 from 4950 to 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.

The quotient of the division is 13196.77, and the remainder is 0.435.