Let's divide 15910531 by 21
The aim is to understand the frequency of 21 in 15910531.

Going ahead to step 1:
1 divided by 21 is 0 with a remainder of 1.
Write down 0 as next digit of the result. 
Result so far: 0.0
The remainder is 1 after subtracting 0 from 1.
Take the next digit (5) from the dividend and append it to 1, then repeat: 15 / 21

Step 2:
21 can be fit into 15 0 times, resulting in a remainder of 15.
The next digit of our result is 0. 
Result so far: 0.0
The remainder is 15 after subtracting 0 from 15.
Take the next digit (9) from the dividend and append it to 15, then repeat: 159 / 21

Advancing to step 3:
The number 21 fits into 159 7 times, leaving a remainder of 12.
Write down 7 as next digit of the result. 
Result so far: 7.0
If we take 147 away from 159, we end up with 12.
Bring next digit (1) of the dividend behind the 12 and repeat the process: 121 / 21

Moving on to step 4:
21 can be fit into 121 5 times, resulting in a remainder of 16.
Use 5 as the next digit of our solution. 
Result so far: 75.0
The remainder is 16 after subtracting 105 from 121.
Include the next digit (0) from the dividend after 16, then repeat: 160 / 21

Step 5:
21 can be fit into 160 7 times, resulting in a remainder of 13.
The number 7 becomes the next digit in our result. 
Result so far: 757.0
The remainder is 13 after subtracting 147 from 160.
Grab the next digit (5) from the dividend, add it to 13, then carry on: 135 / 21

Going ahead to step 6:
The number 21 fits into 135 6 times, leaving a remainder of 9.
Record the quotient 6 as the next digit in the result. 
Result so far: 7576.0
If we subtract 126 from 135, we get 9.
Bring next digit (3) of the dividend behind the 9 and repeat the process: 93 / 21

Moving on to step 7:
93 divided by 21 is 4 with a remainder of 9.
The next digit of our result is 4. 
Result so far: 75764.0
Deduct 84 from 93 and we're left with 9.
Grab the next digit (1) from the dividend, add it to 9, then carry on: 91 / 21

On to step 8:
21 goes into 91 4 times with a remainder of 7.
Write down 4 as next digit of the result. 
Result so far: 757644.0
If we subtract 84 from 91, we 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.

Let's proceed to step 9:
When dividing 70 by 21, we get 3 with a remainder of 7.
Record the quotient 3 as the next digit in the result. 
Result so far: 757644.3
If we take 63 away from 70, we end up 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.

Let's proceed to step 10:
21 can be fit into 70 3 times, resulting in a remainder of 7.
Use 3 as the next digit of our solution. 
Result so far: 757644.33
If we subtract 63 from 70, we 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.

In conclusion, the division gives 757644.33 with a balance of 0.007.