393666 ÷ 21 
We're looking to find how many times 21 goes into 393666.

On to step 1:
If we divide 3 by 21, we get 0 and a remainder of 3.
Put 0 as the next digit of the answer. 
Result so far: 0.0
The remainder is 3 after subtracting 0 from 3.
Include the next digit (9) from the dividend after 3, then repeat: 39 / 21

Step 2:
If we divide 39 by 21, we get 1 and a remainder of 18.
Use 1 as the next digit of our solution. 
Result so far: 1.0
Deduct 21 from 39 and we're left with 18.
Fetch the next digit (3) from the dividend, attach it to 18 and continue: 183 / 21

Let's proceed to step 3:
The number 21 fits into 183 8 times, leaving a remainder of 15.
Record the quotient 8 as the next digit in the result. 
Result so far: 18.0
The remainder is 15 after subtracting 168 from 183.
Bring next digit (6) of the dividend behind the 15 and repeat the process: 156 / 21

Step 4:
21 can be fit into 156 7 times, resulting in a remainder of 9.
Put 7 as the next digit of the answer. 
Result so far: 187.0
Subtracting 147 from 156 leaves us with 9.
Append the next digit (6) from the dividend to 9 and continue with: 96 / 21

Moving on to step 5:
When dividing 96 by 21, we get 4 with a remainder of 12.
Record the quotient 4 as the next digit in the result. 
Result so far: 1874.0
The remainder is 12 after subtracting 84 from 96.
Include the next digit (6) from the dividend after 12, then repeat: 126 / 21

Advancing to step 6:
When dividing 126 by 21, we get 6 with a remainder of 0.
Write down 6 as next digit of the result. 
Result so far: 18746.0
Subtracting 126 from 126 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.

Going ahead to step 7:
21 can be fit into 0 0 times, resulting in a remainder of 0.
Put 0 as the next digit of the answer. 
Result so far: 18746.0
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.

Going ahead to step 8:
0 divided by 21 is 0 with a remainder of 0.
The number 0 becomes the next digit in our result. 
Result so far: 18746.0
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.

After the division, we end up with 18746.0 and a remainder of 0.0.