We divide 351938 by 21
The aim is to understand the frequency of 21 in 351938.

Advancing to step 1:
When dividing 3 by 21, we get 0 with a remainder of 3.
Write down 0 as next digit of the result. 
Result so far: 0.0
The remainder is 3 after subtracting 0 from 3.
Grab the next digit (5) from the dividend, add it to 3, then carry on: 35 / 21

Let's proceed to step 2:
When dividing 35 by 21, we get 1 with a remainder of 14.
Record the quotient 1 as the next digit in the result. 
Result so far: 1.0
Deduct 21 from 35 and we're left with 14.
Include the next digit (1) from the dividend after 14, then repeat: 141 / 21

On to step 3:
If we divide 141 by 21, we get 6 and a remainder of 15.
The next digit of our result is 6. 
Result so far: 16.0
Subtracting 126 from 141 leaves us with 15.
Append the next digit (9) from the dividend to 15 and continue with: 159 / 21

Advancing to step 4:
21 goes into 159 7 times with a remainder of 12.
The next digit of our result is 7. 
Result so far: 167.0
Deduct 147 from 159 and we're left with 12.
Fetch the next digit (3) from the dividend, attach it to 12 and continue: 123 / 21

Step 5:
21 can be fit into 123 5 times, resulting in a remainder of 18.
Use 5 as the next digit of our solution. 
Result so far: 1675.0
If we take 105 away from 123, we end up with 18.
Grab the next digit (8) from the dividend, add it to 18, then carry on: 188 / 21

Moving on to step 6:
When dividing 188 by 21, we get 8 with a remainder of 20.
Record the quotient 8 as the next digit in the result. 
Result so far: 16758.0
If we take 168 away from 188, we end up 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.

Let's proceed to step 7:
200 divided by 21 is 9 with a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 16758.9
Subtracting 189 from 200 leaves us 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 8:
21 can be fit into 110 5 times, resulting in a remainder of 5.
The next digit of our result is 5. 
Result so far: 16758.95
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 9:
50 divided by 21 is 2 with a remainder of 8.
Put 2 as the next digit of the answer. 
Result so far: 16758.952
The remainder is 8 after subtracting 42 from 50.
Since there are no more digits in the dividend, we add a zero to the remainder making it 80, and continue the process.

Step 10:
The number 21 fits into 80 3 times, leaving a remainder of 17.
Use 3 as the next digit of our solution. 
Result so far: 16758.9523
If we take 63 away from 80, we end up with 17.
Since there are no more digits in the dividend, we add a zero to the remainder making it 170, and continue the process.

Let's proceed to step 11:
If we divide 170 by 21, we get 8 and a remainder of 2.
Put 8 as the next digit of the answer. 
Result so far: 16758.95238
If we take 168 away from 170, we end up with 2.
Since there are no more digits in the dividend, we add a zero to the remainder making it 20, and continue the process.

Let's proceed to step 12:
21 goes into 20 0 times with a remainder of 20.
Put 0 as the next digit of the answer. 
Result so far: 16758.95238
Subtracting 0 from 20 leaves us 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.

On to step 13:
21 goes into 200 9 times with a remainder of 11.
Put 9 as the next digit of the answer. 
Result so far: 16758.9523809
If we subtract 189 from 200, we get 11.
Since there are no more digits in the dividend, we add a zero to the remainder making it 110, and continue the process.

Moving on to step 14:
21 can be fit into 110 5 times, resulting in a remainder of 5.
Put 5 as the next digit of the answer. 
Result so far: 16758.95238095
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.

The final outcome is 16758.95238095, with a leftover of 5e-09.