645440 ÷ 21 
We want to figure out the number of times 645440 can be divided by 21.

Moving on to step 1:
When dividing 6 by 21, we get 0 with a remainder of 6.
The next digit of our result is 0. 
Result so far: 0.0
If we subtract 0 from 6, we get 6.
Fetch the next digit (4) from the dividend, attach it to 6 and continue: 64 / 21

Going ahead to step 2:
The number 21 fits into 64 3 times, leaving a remainder of 1.
The next digit of our result is 3. 
Result so far: 3.0
Subtracting 63 from 64 leaves us with 1.
Take the next digit (5) from the dividend and append it to 1, then repeat: 15 / 21

Step 3:
If we divide 15 by 21, we get 0 and a remainder of 15.
The next digit of our result is 0. 
Result so far: 30.0
If we subtract 0 from 15, we get 15.
Grab the next digit (4) from the dividend, add it to 15, then carry on: 154 / 21

Let's proceed to step 4:
If we divide 154 by 21, we get 7 and a remainder of 7.
Put 7 as the next digit of the answer. 
Result so far: 307.0
Subtract 147 from 154 to get 7.
Bring next digit (4) of the dividend behind the 7 and repeat the process: 74 / 21

Let's proceed to step 5:
21 goes into 74 3 times with a remainder of 11.
The number 3 becomes the next digit in our result. 
Result so far: 3073.0
Deduct 63 from 74 and we're left with 11.
Fetch the next digit (0) from the dividend, attach it to 11 and continue: 110 / 21

Step 6:
The number 21 fits into 110 5 times, leaving a remainder of 5.
Write down 5 as next digit of the result. 
Result so far: 30735.0
If we take 105 away from 110, we end up with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Going ahead to step 7:
21 can be fit into 50 2 times, resulting in a remainder of 8.
Write down 2 as next digit of the result. 
Result so far: 30735.2
The remainder is 8 after subtracting 42 from 50.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Let's proceed to step 8:
The number 21 fits into 80 3 times, leaving a remainder of 17.
Put 3 as the next digit of the answer. 
Result so far: 30735.23
If we subtract 63 from 80, we get 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Going ahead to step 9:
21 goes into 170 8 times with a remainder of 2.
The number 8 becomes the next digit in our result. 
Result so far: 30735.238
If we take 168 away from 170, we end up with 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

On to step 10:
20 divided by 21 is 0 with a remainder of 20.
The number 0 becomes the next digit in our result. 
Result so far: 30735.238
Subtract 0 from 20 to get 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

Going ahead to step 11:
When dividing 200 by 21, we get 9 with a remainder of 11.
The next digit of our result is 9. 
Result so far: 30735.23809
Deduct 189 from 200 and we're left with 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Let's proceed to step 12:
When dividing 110 by 21, we get 5 with a remainder of 5.
Put 5 as the next digit of the answer. 
Result so far: 30735.238095
Subtracting 105 from 110 leaves us with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Going ahead to step 13:
21 can be fit into 50 2 times, resulting in a remainder of 8.
Use 2 as the next digit of our solution. 
Result so far: 30735.2380952
Subtracting 42 from 50 leaves us with 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Step 14:
80 divided by 21 is 3 with a remainder of 17.
Use 3 as the next digit of our solution. 
Result so far: 30735.23809523
Subtracting 63 from 80 leaves us with 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

After the division, we end up with 30735.23809523 and a remainder of 1.7e-08.