We look at the division of 72403 by 21 
The aim is to understand the frequency of 21 in 72403.

On to step 1:
21 can be fit into 7 0 times, resulting in a remainder of 7.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Deduct 0 from 7 and we're left with 7.
Take the next digit (2) from the dividend and append it to 7, then repeat: 72 / 21

Step 2:
The number 21 fits into 72 3 times, leaving a remainder of 9.
The next digit of our result is 3. 
Result so far: 3.0
The remainder is 9 after subtracting 63 from 72.
Append the next digit (4) from the dividend to 9 and continue with: 94 / 21

Step 3:
21 can be fit into 94 4 times, resulting in a remainder of 10.
Use 4 as the next digit of our solution. 
Result so far: 34.0
Subtract 84 from 94 to get 10.
Bring next digit (0) of the dividend behind the 10 and repeat the process: 100 / 21

Advancing to step 4:
21 can be fit into 100 4 times, resulting in a remainder of 16.
Record the quotient 4 as the next digit in the result. 
Result so far: 344.0
If we subtract 84 from 100, we get 16.
Bring next digit (3) of the dividend behind the 16 and repeat the process: 163 / 21

Let's proceed to step 5:
163 divided by 21 is 7 with a remainder of 16.
The next digit of our result is 7. 
Result so far: 3447.0
Deduct 147 from 163 and we're left with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Advancing to step 6:
21 goes into 160 7 times with a remainder of 13.
The next digit of our result is 7. 
Result so far: 3447.7
Deduct 147 from 160 and we're left with 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Let's proceed to step 7:
When dividing 130 by 21, we get 6 with a remainder of 4.
The next digit of our result is 6. 
Result so far: 3447.76
If we subtract 126 from 130, we get 4.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

On to step 8:
21 can be fit into 40 1 times, resulting in a remainder of 19.
Use 1 as the next digit of our solution. 
Result so far: 3447.761
The remainder is 19 after subtracting 21 from 40.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

On to step 9:
When dividing 190 by 21, we get 9 with a remainder of 1.
Put 9 as the next digit of the answer. 
Result so far: 3447.7619
If we take 189 away from 190, we end up with 1.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Moving on to step 10:
21 goes into 10 0 times with a remainder of 10.
Record the quotient 0 as the next digit in the result. 
Result so far: 3447.7619
If we subtract 0 from 10, 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.

Going ahead to step 11:
If we divide 100 by 21, we get 4 and a remainder of 16.
Record the quotient 4 as the next digit in the result. 
Result so far: 3447.761904
If we take 84 away from 100, we end up with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Step 12:
21 goes into 160 7 times with a remainder of 13.
Write down 7 as next digit of the result. 
Result so far: 3447.7619047
Subtract 147 from 160 to get 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Moving on to step 13:
21 goes into 130 6 times with a remainder of 4.
Use 6 as the next digit of our solution. 
Result so far: 3447.76190476
Subtract 126 from 130 to get 4.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

The quotient of the division is 3447.76190476, and the remainder is 4e-09.