663915 ÷ 21 
Let's see how many times 21 fits into 663915.

Step 1:
21 can be fit into 6 0 times, resulting in a remainder of 6.
Put 0 as the next digit of the answer. 
Result so far: 0.0
The remainder is 6 after subtracting 0 from 6.
Append the next digit (6) from the dividend to 6 and continue with: 66 / 21

On to step 2:
If we divide 66 by 21, we get 3 and a remainder of 3.
Put 3 as the next digit of the answer. 
Result so far: 3.0
If we take 63 away from 66, we end up with 3.
Fetch the next digit (3) from the dividend, attach it to 3 and continue: 33 / 21

On to step 3:
21 can be fit into 33 1 times, resulting in a remainder of 12.
The number 1 becomes the next digit in our result. 
Result so far: 31.0
The remainder is 12 after subtracting 21 from 33.
Append the next digit (9) from the dividend to 12 and continue with: 129 / 21

Let's proceed to step 4:
21 goes into 129 6 times with a remainder of 3.
Record the quotient 6 as the next digit in the result. 
Result so far: 316.0
If we take 126 away from 129, we end up with 3.
Take the next digit (1) from the dividend and append it to 3, then repeat: 31 / 21

Step 5:
The number 21 fits into 31 1 times, leaving a remainder of 10.
Use 1 as the next digit of our solution. 
Result so far: 3161.0
The remainder is 10 after subtracting 21 from 31.
Fetch the next digit (5) from the dividend, attach it to 10 and continue: 105 / 21

Step 6:
The number 21 fits into 105 5 times, leaving a remainder of 0.
Use 5 as the next digit of our solution. 
Result so far: 31615.0
If we subtract 105 from 105, we 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:
When dividing 0 by 21, we get 0 with a remainder of 0.
The next digit of our result is 0. 
Result so far: 31615.0
Deduct 0 from 0 and we're left 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.

Step 8:
21 goes into 0 0 times with a remainder of 0.
Put 0 as the next digit of the answer. 
Result so far: 31615.0
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.

The final outcome is 31615.0, with a leftover of 0.0.