21399 ÷ 8 = 2674 R7
We're going to perform division on 21399 with 8 as the divisor.

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 (3) of the dividend behind the 5 and repeat the process: 53 / 8

Step 3:
8 goes into 53 6 times with a remainder of 5.
Write down 6 as next digit of of the result. 
Result so far: 26
Subtract 48 from 53 to get 5.
Bring next digit (9) of the dividend behind the 5 and repeat the process: 59 / 8

Step 4:
8 goes into 59 7 times with a remainder of 3.
Write down 7 as next digit of of the result. 
Result so far: 267
Subtract 56 from 59 to get 3.
Bring next digit (9) of the dividend behind the 3 and repeat the process: 39 / 8

Step 5:
8 goes into 39 4 times with a remainder of 7.
Write down 4 as next digit of of the result. 
Result so far: 2674
Subtract 32 from 39 to get 7.

The final result is 2674 with a remainder of 7.