We look at the division of 918904 by 21 
The aim is to understand the frequency of 21 in 918904.

Advancing to step 1:
9 divided by 21 is 0 with a remainder of 9.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
Subtracting 0 from 9 leaves us with 9.
Grab the next digit (1) from the dividend, add it to 9, then carry on: 91 / 21

Step 2:
The number 21 fits into 91 4 times, leaving a remainder of 7.
The number 4 becomes the next digit in our result. 
Result so far: 4.0
Deduct 84 from 91 and we're left with 7.
Grab the next digit (8) from the dividend, add it to 7, then carry on: 78 / 21

Step 3:
When dividing 78 by 21, we get 3 with a remainder of 15.
Write down 3 as next digit of the result. 
Result so far: 43.0
If we take 63 away from 78, we end up with 15.
Take the next digit (9) from the dividend and append it to 15, then repeat: 159 / 21

Going ahead to step 4:
21 goes into 159 7 times with a remainder of 12.
Write down 7 as next digit of the result. 
Result so far: 437.0
The remainder is 12 after subtracting 147 from 159.
Take the next digit (0) from the dividend and append it to 12, then repeat: 120 / 21

Going ahead to step 5:
120 divided by 21 is 5 with a remainder of 15.
Use 5 as the next digit of our solution. 
Result so far: 4375.0
The remainder is 15 after subtracting 105 from 120.
Fetch the next digit (4) from the dividend, attach it to 15 and continue: 154 / 21

Advancing to step 6:
21 goes into 154 7 times with a remainder of 7.
Put 7 as the next digit of the answer. 
Result so far: 43757.0
Deduct 147 from 154 and we're left 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.

Moving on to step 7:
21 goes into 70 3 times with a remainder of 7.
The next digit of our result is 3. 
Result so far: 43757.3
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:
The number 21 fits into 70 3 times, leaving a remainder of 7.
The number 3 becomes the next digit in our result. 
Result so far: 43757.33
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.

After the division, we end up with 43757.33 and a remainder of 0.007.