21901 ÷ 9 = 2433 R4
We're going to perform division on 21901 with 9 as the divisor.

Step 1:
9 goes into 2 0 times with a remainder of 2.
Write down 0 as next digit of of the result. 
Result so far: 0
Subtract 0 from 2 to get 2.
Bring next digit (1) of the dividend behind the 2 and repeat the process: 21 / 9

Step 2:
9 goes into 21 2 times with a remainder of 3.
Write down 2 as next digit of of the result. 
Result so far: 2
Subtract 18 from 21 to get 3.
Bring next digit (9) of the dividend behind the 3 and repeat the process: 39 / 9

Step 3:
9 goes into 39 4 times with a remainder of 3.
Write down 4 as next digit of of the result. 
Result so far: 24
Subtract 36 from 39 to get 3.
Bring next digit (0) of the dividend behind the 3 and repeat the process: 30 / 9

Step 4:
9 goes into 30 3 times with a remainder of 3.
Write down 3 as next digit of of the result. 
Result so far: 243
Subtract 27 from 30 to get 3.
Bring next digit (1) of the dividend behind the 3 and repeat the process: 31 / 9

Step 5:
9 goes into 31 3 times with a remainder of 4.
Write down 3 as next digit of of the result. 
Result so far: 2433
Subtract 27 from 31 to get 4.

The final result is 2433 with a remainder of 4.