We look at the division of 721310 by 81 
The aim is to understand the frequency of 81 in 721310.

On to step 1:
When dividing 7 by 81, we get 0 with a remainder of 7.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Deduct 0 from 7 and we're left with 7.
Append the next digit (2) from the dividend to 7 and continue with: 72 / 81

Step 2:
81 can be fit into 72 0 times, resulting in a remainder of 72.
Write down 0 as next digit of the result. 
Result so far: 0.0
If we take 0 away from 72, we end up with 72.
Append the next digit (1) from the dividend to 72 and continue with: 721 / 81

Step 3:
721 divided by 81 is 8 with a remainder of 73.
The next digit of our result is 8. 
Result so far: 8.0
Deduct 648 from 721 and we're left with 73.
Append the next digit (3) from the dividend to 73 and continue with: 733 / 81

Advancing to step 4:
If we divide 733 by 81, we get 9 and a remainder of 4.
Use 9 as the next digit of our solution. 
Result so far: 89.0
The remainder is 4 after subtracting 729 from 733.
Fetch the next digit (1) from the dividend, attach it to 4 and continue: 41 / 81

On to step 5:
41 divided by 81 is 0 with a remainder of 41.
The number 0 becomes the next digit in our result. 
Result so far: 890.0
If we subtract 0 from 41, we get 41.
Grab the next digit (0) from the dividend, add it to 41, then carry on: 410 / 81

Going ahead to step 6:
410 divided by 81 is 5 with a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 8905.0
If we take 405 away from 410, we end up with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Moving on to step 7:
50 divided by 81 is 0 with a remainder of 50.
The number 0 becomes the next digit in our result. 
Result so far: 8905.0
Subtract 0 from 50 to get 50.
Since there are no more digits in the dividend, we add a zero to the remainder making it 500, and continue the process.

Let's proceed to step 8:
The number 81 fits into 500 6 times, leaving a remainder of 14.
Put 6 as the next digit of the answer. 
Result so far: 8905.06
Subtracting 486 from 500 leaves us with 14.
Since there are no more digits in the dividend, we add a zero to the remainder making it 140, and continue the process.

Advancing to step 9:
81 can be fit into 140 1 times, resulting in a remainder of 59.
The number 1 becomes the next digit in our result. 
Result so far: 8905.061
If we subtract 81 from 140, we get 59.
Since there are no more digits in the dividend, we add a zero to the remainder making it 590, and continue the process.

Advancing to step 10:
81 can be fit into 590 7 times, resulting in a remainder of 23.
Use 7 as the next digit of our solution. 
Result so far: 8905.0617
Subtract 567 from 590 to get 23.
Since there are no more digits in the dividend, we add a zero to the remainder making it 230, and continue the process.

Advancing to step 11:
230 divided by 81 is 2 with a remainder of 68.
Record the quotient 2 as the next digit in the result. 
Result so far: 8905.06172
If we take 162 away from 230, we end up with 68.
Since there are no more digits in the dividend, we add a zero to the remainder making it 680, and continue the process.

Going ahead to step 12:
81 can be fit into 680 8 times, resulting in a remainder of 32.
Use 8 as the next digit of our solution. 
Result so far: 8905.061728
The remainder is 32 after subtracting 648 from 680.
Since there are no more digits in the dividend, we add a zero to the remainder making it 320, and continue the process.

Moving on to step 13:
When dividing 320 by 81, we get 3 with a remainder of 77.
The next digit of our result is 3. 
Result so far: 8905.0617283
The remainder is 77 after subtracting 243 from 320.
Since there are no more digits in the dividend, we add a zero to the remainder making it 770, and continue the process.

On to step 14:
The number 81 fits into 770 9 times, leaving a remainder of 41.
The number 9 becomes the next digit in our result. 
Result so far: 8905.06172839
If we subtract 729 from 770, we get 41.
Since there are no more digits in the dividend, we add a zero to the remainder making it 410, and continue the process.

After the division, we end up with 8905.06172839 and a remainder of 4.1e-08.