640423 divided by 21 
The aim is to understand the frequency of 21 in 640423.

Let's proceed to step 1:
6 divided by 21 is 0 with a remainder of 6.
Put 0 as the next digit of the answer. 
Result so far: 0.0
If we subtract 0 from 6, we get 6.
Grab the next digit (4) from the dividend, add it to 6, then carry on: 64 / 21

Going ahead to step 2:
64 divided by 21 is 3 with a remainder of 1.
Write down 3 as next digit of the result. 
Result so far: 3.0
Subtract 63 from 64 to get 1.
Include the next digit (0) from the dividend after 1, then repeat: 10 / 21

Going ahead to step 3:
The number 21 fits into 10 0 times, leaving a remainder of 10.
The number 0 becomes the next digit in our result. 
Result so far: 30.0
If we take 0 away from 10, we end up with 10.
Append the next digit (4) from the dividend to 10 and continue with: 104 / 21

Moving on to step 4:
When dividing 104 by 21, we get 4 with a remainder of 20.
Write down 4 as next digit of the result. 
Result so far: 304.0
Deduct 84 from 104 and we're left with 20.
Grab the next digit (2) from the dividend, add it to 20, then carry on: 202 / 21

On to step 5:
21 goes into 202 9 times with a remainder of 13.
Put 9 as the next digit of the answer. 
Result so far: 3049.0
Subtracting 189 from 202 leaves us with 13.
Append the next digit (3) from the dividend to 13 and continue with: 133 / 21

On to step 6:
21 can be fit into 133 6 times, resulting in a remainder of 7.
The number 6 becomes the next digit in our result. 
Result so far: 30496.0
Subtracting 126 from 133 leaves us with 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Going ahead to step 7:
70 divided by 21 is 3 with a remainder of 7.
Record the quotient 3 as the next digit in the result. 
Result so far: 30496.3
Subtracting 63 from 70 leaves us with 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

Going ahead to step 8:
21 goes into 70 3 times with a remainder of 7.
Write down 3 as next digit of the result. 
Result so far: 30496.33
Subtract 63 from 70 to get 7.
Since there are no more digits in the dividend, we add a zero to the remainder making it 70, and continue the process.

The quotient of the division is 30496.33, and the remainder is 0.007.