51421015 ÷ 90 
We want to figure out the number of times 51421015 can be divided by 90.

Moving on to step 1:
90 goes into 5 0 times with a remainder of 5.
The next digit of our result is 0. 
Result so far: 0.0
Subtracting 0 from 5 leaves us with 5.
Take the next digit (1) from the dividend and append it to 5, then repeat: 51 / 90

Advancing to step 2:
When dividing 51 by 90, we get 0 with a remainder of 51.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
The remainder is 51 after subtracting 0 from 51.
Append the next digit (4) from the dividend to 51 and continue with: 514 / 90

On to step 3:
90 goes into 514 5 times with a remainder of 64.
The number 5 becomes the next digit in our result. 
Result so far: 5.0
Subtracting 450 from 514 leaves us with 64.
Append the next digit (2) from the dividend to 64 and continue with: 642 / 90

Advancing to step 4:
The number 90 fits into 642 7 times, leaving a remainder of 12.
The number 7 becomes the next digit in our result. 
Result so far: 57.0
If we take 630 away from 642, we end up with 12.
Take the next digit (1) from the dividend and append it to 12, then repeat: 121 / 90

Moving on to step 5:
90 goes into 121 1 times with a remainder of 31.
The number 1 becomes the next digit in our result. 
Result so far: 571.0
If we subtract 90 from 121, we get 31.
Append the next digit (0) from the dividend to 31 and continue with: 310 / 90

On to step 6:
When dividing 310 by 90, we get 3 with a remainder of 40.
Use 3 as the next digit of our solution. 
Result so far: 5713.0
If we take 270 away from 310, we end up with 40.
Grab the next digit (1) from the dividend, add it to 40, then carry on: 401 / 90

Step 7:
90 goes into 401 4 times with a remainder of 41.
The next digit of our result is 4. 
Result so far: 57134.0
Subtracting 360 from 401 leaves us with 41.
Bring next digit (5) of the dividend behind the 41 and repeat the process: 415 / 90

Moving on to step 8:
415 divided by 90 is 4 with a remainder of 55.
The next digit of our result is 4. 
Result so far: 571344.0
If we take 360 away from 415, we end up with 55.
Since there are no more digits in the dividend, we add a zero to the remainder making it 550, and continue the process.

Advancing to step 9:
90 goes into 550 6 times with a remainder of 10.
Use 6 as the next digit of our solution. 
Result so far: 571344.6
If we subtract 540 from 550, we get 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

On to step 10:
90 can be fit into 100 1 times, resulting in a remainder of 10.
Record the quotient 1 as the next digit in the result. 
Result so far: 571344.61
Deduct 90 from 100 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.

Moving on to step 11:
When dividing 100 by 90, we get 1 with a remainder of 10.
The next digit of our result is 1. 
Result so far: 571344.611
If we subtract 90 from 100, we get 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 571344.611, with a leftover of 0.001.