21851 ÷ 8 = 2731 R3
Let's divide 21851 by 8.

Step 1:
8 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 / 8

Step 2:
8 goes into 21 2 times with a remainder of 5.
Write down 2 as next digit of of the result. 
Result so far: 2
Subtract 16 from 21 to get 5.
Bring next digit (8) of the dividend behind the 5 and repeat the process: 58 / 8

Step 3:
8 goes into 58 7 times with a remainder of 2.
Write down 7 as next digit of of the result. 
Result so far: 27
Subtract 56 from 58 to get 2.
Bring next digit (5) of the dividend behind the 2 and repeat the process: 25 / 8

Step 4:
8 goes into 25 3 times with a remainder of 1.
Write down 3 as next digit of of the result. 
Result so far: 273
Subtract 24 from 25 to get 1.
Bring next digit (1) of the dividend behind the 1 and repeat the process: 11 / 8

Step 5:
8 goes into 11 1 times with a remainder of 3.
Write down 1 as next digit of of the result. 
Result so far: 2731
Subtract 8 from 11 to get 3.

The final result is 2731 with a remainder of 3.