We're dividing 421615 by 3 
We want to divide 421615 by 3.

On to step 1:
The number 3 fits into 4 1 times, leaving a remainder of 1.
The next digit of our result is 1. 
Result so far: 1.0
The remainder is 1 after subtracting 3 from 4.
Bring next digit (2) of the dividend behind the 1 and repeat the process: 12 / 3

Let's proceed to step 2:
If we divide 12 by 3, we get 4 and a remainder of 0.
Write down 4 as next digit of the result. 
Result so far: 14.0
Subtracting 12 from 12 leaves us with 0.
Fetch the next digit (1) from the dividend, attach it to 0 and continue: 1 / 3

Advancing to step 3:
If we divide 1 by 3, we get 0 and a remainder of 1.
The number 0 becomes the next digit in our result. 
Result so far: 140.0
The remainder is 1 after subtracting 0 from 1.
Take the next digit (6) from the dividend and append it to 1, then repeat: 16 / 3

Step 4:
If we divide 16 by 3, we get 5 and a remainder of 1.
Put 5 as the next digit of the answer. 
Result so far: 1405.0
Deduct 15 from 16 and we're left with 1.
Bring next digit (1) of the dividend behind the 1 and repeat the process: 11 / 3

Moving on to step 5:
3 goes into 11 3 times with a remainder of 2.
The next digit of our result is 3. 
Result so far: 14053.0
The remainder is 2 after subtracting 9 from 11.
Bring next digit (5) of the dividend behind the 2 and repeat the process: 25 / 3

On to step 6:
If we divide 25 by 3, we get 8 and a remainder of 1.
Record the quotient 8 as the next digit in the result. 
Result so far: 140538.0
Subtracting 24 from 25 leaves us 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.

Moving on to step 7:
When dividing 10 by 3, we get 3 with a remainder of 1.
Use 3 as the next digit of our solution. 
Result so far: 140538.3
If we subtract 9 from 10, we 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.

Step 8:
3 goes into 10 3 times with a remainder of 1.
The number 3 becomes the next digit in our result. 
Result so far: 140538.33
Subtract 9 from 10 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 140538.33 and a remainder of 0.001.