78210090 ÷ 21 
Our goal is to divide 78210090 by 21.

Moving on to step 1:
21 can be fit into 7 0 times, resulting in a remainder of 7.
Write down 0 as next digit of the result. 
Result so far: 0.0
If we subtract 0 from 7, we get 7.
Fetch the next digit (8) from the dividend, attach it to 7 and continue: 78 / 21

Let's proceed to step 2:
The number 21 fits into 78 3 times, leaving a remainder of 15.
The number 3 becomes the next digit in our result. 
Result so far: 3.0
Subtract 63 from 78 to get 15.
Bring next digit (2) of the dividend behind the 15 and repeat the process: 152 / 21

Step 3:
When dividing 152 by 21, we get 7 with a remainder of 5.
Write down 7 as next digit of the result. 
Result so far: 37.0
Subtract 147 from 152 to get 5.
Bring next digit (1) of the dividend behind the 5 and repeat the process: 51 / 21

Going ahead to step 4:
The number 21 fits into 51 2 times, leaving a remainder of 9.
Write down 2 as next digit of the result. 
Result so far: 372.0
The remainder is 9 after subtracting 42 from 51.
Fetch the next digit (0) from the dividend, attach it to 9 and continue: 90 / 21

Advancing to step 5:
90 divided by 21 is 4 with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 3724.0
Subtracting 84 from 90 leaves us with 6.
Fetch the next digit (0) from the dividend, attach it to 6 and continue: 60 / 21

Going ahead to step 6:
60 divided by 21 is 2 with a remainder of 18.
The number 2 becomes the next digit in our result. 
Result so far: 37242.0
The remainder is 18 after subtracting 42 from 60.
Take the next digit (9) from the dividend and append it to 18, then repeat: 189 / 21

Let's proceed to step 7:
The number 21 fits into 189 9 times, leaving a remainder of 0.
Put 9 as the next digit of the answer. 
Result so far: 372429.0
Subtracting 189 from 189 leaves us with 0.
Append the next digit (0) from the dividend to 0 and continue with: 0 / 21

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: 3724290.0
Subtract 0 from 0 to get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

On to step 9:
If we divide 0 by 21, we get 0 and a remainder of 0.
The number 0 becomes the next digit in our result. 
Result so far: 3724290.0
Subtract 0 from 0 to get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

On to step 10:
When dividing 0 by 21, we get 0 with a remainder of 0.
Use 0 as the next digit of our solution. 
Result so far: 3724290.0
If we subtract 0 from 0, we get 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

The final result is 3724290.0 with a remainder of 0.0.