21503 ÷ 6 = 3583 R5
Alright, let's work through the division of 21503 by 6 step by step.

Step 1:
6 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 / 6

Step 2:
6 goes into 21 3 times with a remainder of 3.
Write down 3 as next digit of of the result. 
Result so far: 3
Subtract 18 from 21 to get 3.
Bring next digit (5) of the dividend behind the 3 and repeat the process: 35 / 6

Step 3:
6 goes into 35 5 times with a remainder of 5.
Write down 5 as next digit of of the result. 
Result so far: 35
Subtract 30 from 35 to get 5.
Bring next digit (0) of the dividend behind the 5 and repeat the process: 50 / 6

Step 4:
6 goes into 50 8 times with a remainder of 2.
Write down 8 as next digit of of the result. 
Result so far: 358
Subtract 48 from 50 to get 2.
Bring next digit (3) of the dividend behind the 2 and repeat the process: 23 / 6

Step 5:
6 goes into 23 3 times with a remainder of 5.
Write down 3 as next digit of of the result. 
Result so far: 3583
Subtract 18 from 23 to get 5.

The final result is 3583 with a remainder of 5.