Let's divide 51215225 by 399
We want to figure out the number of times 51215225 can be divided by 399.

Step 1:
399 goes into 5 0 times with a remainder of 5.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
The remainder is 5 after subtracting 0 from 5.
Include the next digit (1) from the dividend after 5, then repeat: 51 / 399

Moving on to step 2:
51 divided by 399 is 0 with a remainder of 51.
Use 0 as the next digit of our solution. 
Result so far: 0.0
If we take 0 away from 51, we end up with 51.
Include the next digit (2) from the dividend after 51, then repeat: 512 / 399

Advancing to step 3:
399 goes into 512 1 times with a remainder of 113.
Use 1 as the next digit of our solution. 
Result so far: 1.0
Deduct 399 from 512 and we're left with 113.
Bring next digit (1) of the dividend behind the 113 and repeat the process: 1131 / 399

Moving on to step 4:
The number 399 fits into 1131 2 times, leaving a remainder of 333.
Put 2 as the next digit of the answer. 
Result so far: 12.0
Deduct 798 from 1131 and we're left with 333.
Bring next digit (5) of the dividend behind the 333 and repeat the process: 3335 / 399

Advancing to step 5:
When dividing 3335 by 399, we get 8 with a remainder of 143.
The next digit of our result is 8. 
Result so far: 128.0
Subtract 3192 from 3335 to get 143.
Grab the next digit (2) from the dividend, add it to 143, then carry on: 1432 / 399

Moving on to step 6:
1432 divided by 399 is 3 with a remainder of 235.
Write down 3 as next digit of the result. 
Result so far: 1283.0
Deduct 1197 from 1432 and we're left with 235.
Append the next digit (2) from the dividend to 235 and continue with: 2352 / 399

On to step 7:
399 goes into 2352 5 times with a remainder of 357.
The number 5 becomes the next digit in our result. 
Result so far: 12835.0
Deduct 1995 from 2352 and we're left with 357.
Take the next digit (5) from the dividend and append it to 357, then repeat: 3575 / 399

On to step 8:
399 can be fit into 3575 8 times, resulting in a remainder of 383.
The next digit of our result is 8. 
Result so far: 128358.0
If we take 3192 away from 3575, we end up with 383.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3830, and continue the process.

Going ahead to step 9:
3830 divided by 399 is 9 with a remainder of 239.
Use 9 as the next digit of our solution. 
Result so far: 128358.9
Subtracting 3591 from 3830 leaves us with 239.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2390, and continue the process.

Let's proceed to step 10:
2390 divided by 399 is 5 with a remainder of 395.
Record the quotient 5 as the next digit in the result. 
Result so far: 128358.95
Subtract 1995 from 2390 to get 395.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3950, and continue the process.

Step 11:
If we divide 3950 by 399, we get 9 and a remainder of 359.
Record the quotient 9 as the next digit in the result. 
Result so far: 128358.959
If we subtract 3591 from 3950, we get 359.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3590, and continue the process.

Going ahead to step 12:
399 goes into 3590 8 times with a remainder of 398.
The number 8 becomes the next digit in our result. 
Result so far: 128358.9598
If we take 3192 away from 3590, we end up with 398.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3980, and continue the process.

Let's proceed to step 13:
3980 divided by 399 is 9 with a remainder of 389.
The number 9 becomes the next digit in our result. 
Result so far: 128358.95989
The remainder is 389 after subtracting 3591 from 3980.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3890, and continue the process.

Advancing to step 14:
399 can be fit into 3890 9 times, resulting in a remainder of 299.
Record the quotient 9 as the next digit in the result. 
Result so far: 128358.959899
Deduct 3591 from 3890 and we're left with 299.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2990, and continue the process.

Our division results in 128358.959899 with a remaining 2.99e-05.