Solution : Three Prisoners and King Logic Riddle

You can read the original puzzle here : Three Prisoners and King Logical Riddle

Now the solution:
The color at back of released prisoner was White. Logic is as follows.

Let us give numbers to prisoners as 1, 2 and 3 .

Number 2 being the prisoner who gave correct answer and was released . Now since no. 1 and no.3 has white discs on their back so only two possible combinations are

• White(no.1)-black(no.2)-white(no.3) 
• White(no.1)-white(no.2)-white(no.2) 

 Taking first case as White ,black and White , now think over what would no.1 prisoner think?
He would think that if he had black disc at his back then no. 3 prisoner who must be seeing two blacks now (one at back of no.2 and other at back of no.1) will be sure of disc at his own back as white because there were only two black discs.
 But no.1 would see that no.2 prisoner is confused as he is not going to guard , so there must be a white at back of number 1.

 Hence if the combination was White – Black- white then no.1 would be sure of his White color.

 But as till 30 minutes no one went to the guard , this made clear to no.2 that the combination was NOT White- Black- White , hence it was clear that the combination was White- White- White.

Discuss the solution in comments.  :)