25122100 ÷ 870 
We want to figure out the number of times 25122100 can be divided by 870.

Moving on to step 1:
When dividing 2 by 870, we get 0 with a remainder of 2.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Subtract 0 from 2 to get 2.
Grab the next digit (5) from the dividend, add it to 2, then carry on: 25 / 870

Advancing to step 2:
870 goes into 25 0 times with a remainder of 25.
Use 0 as the next digit of our solution. 
Result so far: 0.0
Subtract 0 from 25 to get 25.
Fetch the next digit (1) from the dividend, attach it to 25 and continue: 251 / 870

On to step 3:
When dividing 251 by 870, we get 0 with a remainder of 251.
Use 0 as the next digit of our solution. 
Result so far: 0.0
The remainder is 251 after subtracting 0 from 251.
Grab the next digit (2) from the dividend, add it to 251, then carry on: 2512 / 870

Advancing to step 4:
870 goes into 2512 2 times with a remainder of 772.
Put 2 as the next digit of the answer. 
Result so far: 2.0
Deduct 1740 from 2512 and we're left with 772.
Fetch the next digit (2) from the dividend, attach it to 772 and continue: 7722 / 870

Going ahead to step 5:
870 goes into 7722 8 times with a remainder of 762.
Record the quotient 8 as the next digit in the result. 
Result so far: 28.0
Subtract 6960 from 7722 to get 762.
Bring next digit (1) of the dividend behind the 762 and repeat the process: 7621 / 870

Step 6:
If we divide 7621 by 870, we get 8 and a remainder of 661.
Put 8 as the next digit of the answer. 
Result so far: 288.0
If we take 6960 away from 7621, we end up with 661.
Bring next digit (0) of the dividend behind the 661 and repeat the process: 6610 / 870

Going ahead to step 7:
870 can be fit into 6610 7 times, resulting in a remainder of 520.
Use 7 as the next digit of our solution. 
Result so far: 2887.0
Subtracting 6090 from 6610 leaves us with 520.
Fetch the next digit (0) from the dividend, attach it to 520 and continue: 5200 / 870

Let's proceed to step 8:
870 goes into 5200 5 times with a remainder of 850.
Put 5 as the next digit of the answer. 
Result so far: 28875.0
Deduct 4350 from 5200 and we're left with 850.
Since there are no more digits in the dividend, we add a zero to the remainder making it 8500, and continue the process.

Moving on to step 9:
870 can be fit into 8500 9 times, resulting in a remainder of 670.
Use 9 as the next digit of our solution. 
Result so far: 28875.9
Subtract 7830 from 8500 to get 670.
Since there are no more digits in the dividend, we add a zero to the remainder making it 6700, and continue the process.

Let's proceed to step 10:
When dividing 6700 by 870, we get 7 with a remainder of 610.
The number 7 becomes the next digit in our result. 
Result so far: 28875.97
If we subtract 6090 from 6700, we get 610.
Since there are no more digits in the dividend, we add a zero to the remainder making it 6100, and continue the process.

On to step 11:
The number 870 fits into 6100 7 times, leaving a remainder of 10.
Use 7 as the next digit of our solution. 
Result so far: 28875.977
Deduct 6090 from 6100 and we're left with 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

The final outcome is 28875.977, with a leftover of 0.001.