We divide 78478529 by 21
The aim is to understand the frequency of 21 in 78478529.

Going ahead to step 1:
7 divided by 21 is 0 with a remainder of 7.
The number 0 becomes the next digit in our result. 
Result so far: 0.0
If we take 0 away from 7, we end up with 7.
Bring next digit (8) of the dividend behind the 7 and repeat the process: 78 / 21

Advancing to step 2:
If we divide 78 by 21, we get 3 and a remainder of 15.
The next digit of our result is 3. 
Result so far: 3.0
Deduct 63 from 78 and we're left with 15.
Append the next digit (4) from the dividend to 15 and continue with: 154 / 21

On to step 3:
21 can be fit into 154 7 times, resulting in a remainder of 7.
Record the quotient 7 as the next digit in the result. 
Result so far: 37.0
If we take 147 away from 154, we end up with 7.
Append the next digit (7) from the dividend to 7 and continue with: 77 / 21

Let's proceed to step 4:
If we divide 77 by 21, we get 3 and a remainder of 14.
Write down 3 as next digit of the result. 
Result so far: 373.0
If we subtract 63 from 77, we get 14.
Append the next digit (8) from the dividend to 14 and continue with: 148 / 21

Let's proceed to step 5:
When dividing 148 by 21, we get 7 with a remainder of 1.
Put 7 as the next digit of the answer. 
Result so far: 3737.0
Subtracting 147 from 148 leaves us with 1.
Take the next digit (5) from the dividend and append it to 1, then repeat: 15 / 21

Step 6:
21 can be fit into 15 0 times, resulting in a remainder of 15.
Record the quotient 0 as the next digit in the result. 
Result so far: 37370.0
Subtracting 0 from 15 leaves us with 15.
Bring next digit (2) of the dividend behind the 15 and repeat the process: 152 / 21

Moving on to step 7:
If we divide 152 by 21, we get 7 and a remainder of 5.
Record the quotient 7 as the next digit in the result. 
Result so far: 373707.0
If we subtract 147 from 152, we get 5.
Fetch the next digit (9) from the dividend, attach it to 5 and continue: 59 / 21

On to step 8:
When dividing 59 by 21, we get 2 with a remainder of 17.
The number 2 becomes the next digit in our result. 
Result so far: 3737072.0
Subtracting 42 from 59 leaves us 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.

On to step 9:
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: 3737072.8
Subtracting 168 from 170 leaves us 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.

Advancing to step 10:
20 divided by 21 is 0 with a remainder of 20.
Use 0 as the next digit of our solution. 
Result so far: 3737072.8
Subtract 0 from 20 to get 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 11:
The number 21 fits into 200 9 times, leaving a remainder of 11.
Record the quotient 9 as the next digit in the result. 
Result so far: 3737072.809
Subtract 189 from 200 to 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.

On to step 12:
If we divide 110 by 21, we get 5 and a remainder of 5.
The next digit of our result is 5. 
Result so far: 3737072.8095
Subtract 105 from 110 to get 5.
Since there are no more digits in the dividend, we add a zero to the remainder making it 50, and continue the process.

Step 13:
If we divide 50 by 21, we get 2 and a remainder of 8.
Write down 2 as next digit of the result. 
Result so far: 3737072.80952
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.

On to step 14:
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: 3737072.809523
The remainder is 17 after subtracting 63 from 80.
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 15:
The number 21 fits into 170 8 times, leaving a remainder of 2.
Record the quotient 8 as the next digit in the result. 
Result so far: 3737072.8095238
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.

Moving on to step 16:
The number 21 fits into 20 0 times, leaving a remainder of 20.
Use 0 as the next digit of our solution. 
Result so far: 3737072.8095238
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.

The final outcome is 3737072.8095238, with a leftover of 2e-08.