We look at the division of 851137 by 21 
Let's see how many times 21 fits into 851137.

On to step 1:
8 divided by 21 is 0 with a remainder of 8.
Write down 0 as next digit of the result. 
Result so far: 0.0
Subtract 0 from 8 to get 8.
Bring next digit (5) of the dividend behind the 8 and repeat the process: 85 / 21

Going ahead to step 2:
If we divide 85 by 21, we get 4 and a remainder of 1.
The next digit of our result is 4. 
Result so far: 4.0
The remainder is 1 after subtracting 84 from 85.
Append the next digit (1) from the dividend to 1 and continue with: 11 / 21

Advancing to step 3:
21 can be fit into 11 0 times, resulting in a remainder of 11.
Put 0 as the next digit of the answer. 
Result so far: 40.0
If we subtract 0 from 11, we get 11.
Take the next digit (1) from the dividend and append it to 11, then repeat: 111 / 21

Step 4:
If we divide 111 by 21, we get 5 and a remainder of 6.
Write down 5 as next digit of the result. 
Result so far: 405.0
Subtracting 105 from 111 leaves us with 6.
Fetch the next digit (3) from the dividend, attach it to 6 and continue: 63 / 21

Let's proceed to step 5:
When dividing 63 by 21, we get 3 with a remainder of 0.
Use 3 as the next digit of our solution. 
Result so far: 4053.0
The remainder is 0 after subtracting 63 from 63.
Append the next digit (7) from the dividend to 0 and continue with: 7 / 21

On to step 6:
21 goes into 7 0 times with a remainder of 7.
Record the quotient 0 as the next digit in the result. 
Result so far: 40530.0
Deduct 0 from 7 and we're left with 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Going ahead to step 7:
When dividing 70 by 21, we get 3 with a remainder of 7.
The number 3 becomes the next digit in our result. 
Result so far: 40530.3
Subtract 63 from 70 to get 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Going ahead to step 8:
21 goes into 70 3 times with a remainder of 7.
Write down 3 as next digit of the result. 
Result so far: 40530.33
If we subtract 63 from 70, we get 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

The final outcome is 40530.33, with a leftover of 0.007.