878623 divided by 21 
The aim is to understand the frequency of 21 in 878623.

Advancing to step 1:
When dividing 8 by 21, we get 0 with a remainder of 8.
Record the quotient 0 as the next digit in the result. 
Result so far: 0.0
The remainder is 8 after subtracting 0 from 8.
Bring next digit (7) of the dividend behind the 8 and repeat the process: 87 / 21

On to step 2:
21 goes into 87 4 times with a remainder of 3.
Use 4 as the next digit of our solution. 
Result so far: 4.0
If we take 84 away from 87, we end up with 3.
Append the next digit (8) from the dividend to 3 and continue with: 38 / 21

Step 3:
When dividing 38 by 21, we get 1 with a remainder of 17.
The number 1 becomes the next digit in our result. 
Result so far: 41.0
Subtract 21 from 38 to get 17.
Grab the next digit (6) from the dividend, add it to 17, then carry on: 176 / 21

Step 4:
21 goes into 176 8 times with a remainder of 8.
Put 8 as the next digit of the answer. 
Result so far: 418.0
Subtract 168 from 176 to get 8.
Grab the next digit (2) from the dividend, add it to 8, then carry on: 82 / 21

Advancing to step 5:
The number 21 fits into 82 3 times, leaving a remainder of 19.
Record the quotient 3 as the next digit in the result. 
Result so far: 4183.0
Deduct 63 from 82 and we're left with 19.
Grab the next digit (3) from the dividend, add it to 19, then carry on: 193 / 21

Step 6:
21 goes into 193 9 times with a remainder of 4.
Record the quotient 9 as the next digit in the result. 
Result so far: 41839.0
Subtract 189 from 193 to get 4.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

Advancing to step 7:
When dividing 40 by 21, we get 1 with a remainder of 19.
Record the quotient 1 as the next digit in the result. 
Result so far: 41839.1
If we subtract 21 from 40, we get 19.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

On to step 8:
21 can be fit into 190 9 times, resulting in a remainder of 1.
Use 9 as the next digit of our solution. 
Result so far: 41839.19
If we take 189 away from 190, we end up with 1.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

Let's proceed to step 9:
21 can be fit into 10 0 times, resulting in a remainder of 10.
Write down 0 as next digit of the result. 
Result so far: 41839.19
Deduct 0 from 10 and we're left with 10.
Since there are no more digits in the dividend, we add a zero to the remainder making it 100, and continue the process.

Moving on to step 10:
If we divide 100 by 21, we get 4 and a remainder of 16.
Put 4 as the next digit of the answer. 
Result so far: 41839.1904
If we take 84 away from 100, we end up with 16.
Since there are no more digits in the dividend, we add a zero to the remainder making it 160, and continue the process.

Moving on to step 11:
If we divide 160 by 21, we get 7 and a remainder of 13.
The number 7 becomes the next digit in our result. 
Result so far: 41839.19047
Subtract 147 from 160 to get 13.
Since there are no more digits in the dividend, we add a zero to the remainder making it 130, and continue the process.

Moving on to step 12:
21 goes into 130 6 times with a remainder of 4.
The next digit of our result is 6. 
Result so far: 41839.190476
Subtract 126 from 130 to get 4.
Since there are no more digits in the dividend, we add a zero to the remainder making it 40, and continue the process.

Moving on to step 13:
21 goes into 40 1 times with a remainder of 19.
The number 1 becomes the next digit in our result. 
Result so far: 41839.1904761
Subtracting 21 from 40 leaves us with 19.
Since there are no more digits in the dividend, we add a zero to the remainder making it 190, and continue the process.

Going ahead to step 14:
21 can be fit into 190 9 times, resulting in a remainder of 1.
The next digit of our result is 9. 
Result so far: 41839.19047619
Subtract 189 from 190 to get 1.
Since there are no more digits in the dividend, we add a zero to the remainder making it 10, and continue the process.

After the division, we end up with 41839.19047619 and a remainder of 1e-09.