We look at the division of 366051 by 21 
The aim is to understand the frequency of 21 in 366051.

Step 1:
When dividing 3 by 21, we get 0 with a remainder of 3.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
Subtract 0 from 3 to get 3.
Include the next digit (6) from the dividend after 3, then repeat: 36 / 21

Moving on to step 2:
If we divide 36 by 21, we get 1 and a remainder of 15.
Use 1 as the next digit of our solution. 
Result so far: 1.0
Deduct 21 from 36 and we're left with 15.
Take the next digit (6) from the dividend and append it to 15, then repeat: 156 / 21

Moving on to step 3:
21 goes into 156 7 times with a remainder of 9.
Use 7 as the next digit of our solution. 
Result so far: 17.0
Subtract 147 from 156 to get 9.
Grab the next digit (0) from the dividend, add it to 9, then carry on: 90 / 21

Moving on to step 4:
21 goes into 90 4 times with a remainder of 6.
Write down 4 as next digit of the result. 
Result so far: 174.0
The remainder is 6 after subtracting 84 from 90.
Append the next digit (5) from the dividend to 6 and continue with: 65 / 21

Moving on to step 5:
65 divided by 21 is 3 with a remainder of 2.
The number 3 becomes the next digit in our result. 
Result so far: 1743.0
Subtract 63 from 65 to get 2.
Append the next digit (1) from the dividend to 2 and continue with: 21 / 21

Let's proceed to step 6:
21 divided by 21 is 1 with a remainder of 0.
The next digit of our result is 1. 
Result so far: 17431.0
Subtracting 21 from 21 leaves us with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Let's proceed to step 7:
The number 21 fits into 0 0 times, leaving a remainder of 0.
Record the quotient 0 as the next digit in the result. 
Result so far: 17431.0
If we take 0 away from 0, we end up with 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Let's proceed to step 8:
0 divided by 21 is 0 with a remainder of 0.
Write down 0 as next digit of the result. 
Result so far: 17431.0
The remainder is 0 after subtracting 0 from 0.
Since there are no more digits in the dividend, we add a zero to the remainder making it 0, and continue the process.

Our division results in 17431.0 with a remaining 0.0.