687413 ÷ 21 
The aim is to understand the frequency of 21 in 687413.

Step 1:
The number 21 fits into 6 0 times, leaving a remainder of 6.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we subtract 0 from 6, we get 6.
Include the next digit (8) from the dividend after 6, then repeat: 68 / 21

Step 2:
If we divide 68 by 21, we get 3 and a remainder of 5.
The number 3 becomes the next digit in our result. 
Result so far: 3.0
Subtracting 63 from 68 leaves us with 5.
Fetch the next digit (7) from the dividend, attach it to 5 and continue: 57 / 21

On to step 3:
57 divided by 21 is 2 with a remainder of 15.
Record the quotient 2 as the next digit in the result. 
Result so far: 32.0
The remainder is 15 after subtracting 42 from 57.
Grab the next digit (4) from the dividend, add it to 15, then carry on: 154 / 21

Advancing to step 4:
When dividing 154 by 21, we get 7 with a remainder of 7.
Put 7 as the next digit of the answer. 
Result so far: 327.0
If we subtract 147 from 154, we get 7.
Bring next digit (1) of the dividend behind the 7 and repeat the process: 71 / 21

Let's proceed to step 5:
21 goes into 71 3 times with a remainder of 8.
The number 3 becomes the next digit in our result. 
Result so far: 3273.0
Subtract 63 from 71 to get 8.
Include the next digit (3) from the dividend after 8, then repeat: 83 / 21

Going ahead to step 6:
If we divide 83 by 21, we get 3 and a remainder of 20.
Record the quotient 3 as the next digit in the result. 
Result so far: 32733.0
Deduct 63 from 83 and we're left with 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

Step 7:
21 can be fit into 200 9 times, resulting in a remainder of 11.
Record the quotient 9 as the next digit in the result. 
Result so far: 32733.9
Subtracting 189 from 200 leaves us with 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Going ahead to step 8:
When dividing 110 by 21, we get 5 with a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 32733.95
Subtract 105 from 110 to get 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

On to step 9:
When dividing 50 by 21, we get 2 with a remainder of 8.
Put 2 as the next digit of the answer. 
Result so far: 32733.952
If we subtract 42 from 50, we get 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Advancing to step 10:
21 can be fit into 80 3 times, resulting in a remainder of 17.
Use 3 as the next digit of our solution. 
Result so far: 32733.9523
Subtracting 63 from 80 leaves us with 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

On to step 11:
When dividing 170 by 21, we get 8 with a remainder of 2.
Put 8 as the next digit of the answer. 
Result so far: 32733.95238
If we subtract 168 from 170, we get 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

On to step 12:
The number 21 fits into 20 0 times, leaving a remainder of 20.
The number 0 becomes the next digit in our result. 
Result so far: 32733.95238
Subtract 0 from 20 to get 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

On to step 13:
21 can be fit into 200 9 times, resulting in a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 32733.9523809
If we subtract 189 from 200, we get 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Step 14:
110 divided by 21 is 5 with a remainder of 5.
Write down 5 as next digit of the result. 
Result so far: 32733.95238095
Subtract 105 from 110 to get 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

The final result is 32733.95238095 with a remainder of 5e-09.