We're dividing 66290420 by 21 
We want to figure out the number of times 66290420 can be divided by 21.

Step 1:
21 can be fit into 6 0 times, resulting in a remainder of 6.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we take 0 away from 6, we end up with 6.
Append the next digit (6) from the dividend to 6 and continue with: 66 / 21

On to step 2:
66 divided by 21 is 3 with a remainder of 3.
The number 3 becomes the next digit in our result. 
Result so far: 3.0
Subtracting 63 from 66 leaves us with 3.
Fetch the next digit (2) from the dividend, attach it to 3 and continue: 32 / 21

On to step 3:
The number 21 fits into 32 1 times, leaving a remainder of 11.
Use 1 as the next digit of our solution. 
Result so far: 31.0
If we take 21 away from 32, we end up with 11.
Include the next digit (9) from the dividend after 11, then repeat: 119 / 21

On to step 4:
21 goes into 119 5 times with a remainder of 14.
The number 5 becomes the next digit in our result. 
Result so far: 315.0
If we take 105 away from 119, we end up with 14.
Fetch the next digit (0) from the dividend, attach it to 14 and continue: 140 / 21

Step 5:
21 goes into 140 6 times with a remainder of 14.
The next digit of our result is 6. 
Result so far: 3156.0
The remainder is 14 after subtracting 126 from 140.
Take the next digit (4) from the dividend and append it to 14, then repeat: 144 / 21

Advancing to step 6:
If we divide 144 by 21, we get 6 and a remainder of 18.
The next digit of our result is 6. 
Result so far: 31566.0
Subtracting 126 from 144 leaves us with 18.
Bring next digit (2) of the dividend behind the 18 and repeat the process: 182 / 21

Going ahead to step 7:
If we divide 182 by 21, we get 8 and a remainder of 14.
Put 8 as the next digit of the answer. 
Result so far: 315668.0
Subtract 168 from 182 to get 14.
Grab the next digit (0) from the dividend, add it to 14, then carry on: 140 / 21

On to step 8:
140 divided by 21 is 6 with a remainder of 14.
Put 6 as the next digit of the answer. 
Result so far: 3156686.0
If we subtract 126 from 140, we get 14.
Since there are no more digits in the dividend, we add a zero to the remainder making it 140, and continue the process.

Step 9:
When dividing 140 by 21, we get 6 with a remainder of 14.
Put 6 as the next digit of the answer. 
Result so far: 3156686.6
Deduct 126 from 140 and we're left with 14.
Since there are no more digits in the dividend, we add a zero to the remainder making it 140, and continue the process.

Advancing to step 10:
140 divided by 21 is 6 with a remainder of 14.
The number 6 becomes the next digit in our result. 
Result so far: 3156686.66
Subtracting 126 from 140 leaves us with 14.
Since there are no more digits in the dividend, we add a zero to the remainder making it 140, and continue the process.

The final outcome is 3156686.66, with a leftover of 0.014.