We're dividing 1981836 by 645 
The aim is to understand the frequency of 645 in 1981836.

On to step 1:
The number 645 fits into 1 0 times, leaving a remainder of 1.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
If we subtract 0 from 1, we get 1.
Take the next digit (9) from the dividend and append it to 1, then repeat: 19 / 645

Going ahead to step 2:
When dividing 19 by 645, we get 0 with a remainder of 19.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
The remainder is 19 after subtracting 0 from 19.
Include the next digit (8) from the dividend after 19, then repeat: 198 / 645

Going ahead to step 3:
The number 645 fits into 198 0 times, leaving a remainder of 198.
The next digit of our result is 0. 
Result so far: 0.0
Subtract 0 from 198 to get 198.
Grab the next digit (1) from the dividend, add it to 198, then carry on: 1981 / 645

Advancing to step 4:
When dividing 1981 by 645, we get 3 with a remainder of 46.
The number 3 becomes the next digit in our result. 
Result so far: 3.0
If we take 1935 away from 1981, we end up with 46.
Grab the next digit (8) from the dividend, add it to 46, then carry on: 468 / 645

On to step 5:
645 can be fit into 468 0 times, resulting in a remainder of 468.
Record the quotient 0 as the next digit in the result. 
Result so far: 30.0
If we take 0 away from 468, we end up with 468.
Take the next digit (3) from the dividend and append it to 468, then repeat: 4683 / 645

Moving on to step 6:
If we divide 4683 by 645, we get 7 and a remainder of 168.
Put 7 as the next digit of the answer. 
Result so far: 307.0
Subtracting 4515 from 4683 leaves us with 168.
Take the next digit (6) from the dividend and append it to 168, then repeat: 1686 / 645

On to step 7:
645 goes into 1686 2 times with a remainder of 396.
The next digit of our result is 2. 
Result so far: 3072.0
If we take 1290 away from 1686, we end up with 396.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3960, and continue the process.

Let's proceed to step 8:
When dividing 3960 by 645, we get 6 with a remainder of 90.
The next digit of our result is 6. 
Result so far: 3072.6
If we subtract 3870 from 3960, we get 90.
Since there are no more digits in the dividend, we add a zero to the remainder making it 900, and continue the process.

On to step 9:
When dividing 900 by 645, we get 1 with a remainder of 255.
Record the quotient 1 as the next digit in the result. 
Result so far: 3072.61
Subtract 645 from 900 to get 255.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2550, and continue the process.

On to step 10:
645 goes into 2550 3 times with a remainder of 615.
The next digit of our result is 3. 
Result so far: 3072.613
If we take 1935 away from 2550, we end up with 615.
Since there are no more digits in the dividend, we add a zero to the remainder making it 6150, and continue the process.

On to step 11:
When dividing 6150 by 645, we get 9 with a remainder of 345.
Put 9 as the next digit of the answer. 
Result so far: 3072.6139
Subtract 5805 from 6150 to get 345.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3450, and continue the process.

Advancing to step 12:
The number 645 fits into 3450 5 times, leaving a remainder of 225.
Use 5 as the next digit of our solution. 
Result so far: 3072.61395
Deduct 3225 from 3450 and we're left with 225.
Since there are no more digits in the dividend, we add a zero to the remainder making it 2250, and continue the process.

Moving on to step 13:
645 goes into 2250 3 times with a remainder of 315.
The next digit of our result is 3. 
Result so far: 3072.613953
The remainder is 315 after subtracting 1935 from 2250.
Since there are no more digits in the dividend, we add a zero to the remainder making it 3150, and continue the process.

Advancing to step 14:
3150 divided by 645 is 4 with a remainder of 570.
Put 4 as the next digit of the answer. 
Result so far: 3072.6139534
Subtracting 2580 from 3150 leaves us with 570.
Since there are no more digits in the dividend, we add a zero to the remainder making it 5700, and continue the process.

Step 15:
The number 645 fits into 5700 8 times, leaving a remainder of 540.
The next digit of our result is 8. 
Result so far: 3072.61395348
If we subtract 5160 from 5700, we get 540.
Since there are no more digits in the dividend, we add a zero to the remainder making it 5400, and continue the process.

The final outcome is 3072.61395348, with a leftover of 5.4e-07.