3491 divided by 21 
The aim is to understand the frequency of 21 in 3491.

Going ahead to 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
The remainder is 3 after subtracting 0 from 3.
Append the next digit (4) from the dividend to 3 and continue with: 34 / 21

Moving on to step 2:
The number 21 fits into 34 1 times, leaving a remainder of 13.
Use 1 as the next digit of our solution. 
Result so far: 1.0
Subtract 21 from 34 to get 13.
Append the next digit (9) from the dividend to 13 and continue with: 139 / 21

Moving on to step 3:
When dividing 139 by 21, we get 6 with a remainder of 13.
Record the quotient 6 as the next digit in the result. 
Result so far: 16.0
Deduct 126 from 139 and we're left with 13.
Bring next digit (1) of the dividend behind the 13 and repeat the process: 131 / 21

Going ahead to step 4:
When dividing 131 by 21, we get 6 with a remainder of 5.
Put 6 as the next digit of the answer. 
Result so far: 166.0
Subtracting 126 from 131 leaves us with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Let's proceed to step 5:
The number 21 fits into 50 2 times, leaving a remainder of 8.
The number 2 becomes the next digit in our result. 
Result so far: 166.2
If we take 42 away from 50, we end up with 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Advancing to step 6:
21 can be fit into 80 3 times, resulting in a remainder of 17.
Use 3 as the next digit of our solution. 
Result so far: 166.23
If we subtract 63 from 80, we get 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

On to step 7:
21 can be fit into 170 8 times, resulting in a remainder of 2.
Use 8 as the next digit of our solution. 
Result so far: 166.238
The remainder is 2 after subtracting 168 from 170.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Going ahead to step 8:
21 can be fit into 20 0 times, resulting in a remainder of 20.
The number 0 becomes the next digit in our result. 
Result so far: 166.238
Deduct 0 from 20 and we're left with 20.
Since there are no more digits in the dividend, we add a zero to the remainder making it 200, and continue the process.

Moving on to step 9:
21 goes into 200 9 times with a remainder of 11.
The number 9 becomes the next digit in our result. 
Result so far: 166.23809
If we take 189 away from 200, we end up with 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

On to step 10:
When dividing 110 by 21, we get 5 with a remainder of 5.
Record the quotient 5 as the next digit in the result. 
Result so far: 166.238095
If we take 105 away from 110, we end up with 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Going ahead to step 11:
50 divided by 21 is 2 with a remainder of 8.
Use 2 as the next digit of our solution. 
Result so far: 166.2380952
Subtract 42 from 50 to get 8.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Moving on to step 12:
21 can be fit into 80 3 times, resulting in a remainder of 17.
Put 3 as the next digit of the answer. 
Result so far: 166.23809523
Subtract 63 from 80 to get 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

After the division, we end up with 166.23809523 and a remainder of 1.7e-08.