There are four balls in a hat: a blue one, a white one, and two red ones. Now I draw simultaneously two balls, look at them, and announce that at least one of them is red.
What is the chance that the other is red as well?
(In reply to There is only one correct answer !....SPOILER
by Ady TZIDON)
I do not fault you for grading the puzzle as D2. It is a basic problem in conditional probability, much like the classic Monty Hall problem. If you go searching for discussions about that problem you will find lots of threads debating if the right answer is 1/3 or 1/2.
The difficulty of the problem is not the issue. It is how people interpret and understand the concept behind the puzzle that generates so much discussion. Just look at how much talk our dwindling community has generated. I think every regular has weighed in on this one. You can clearly see the two sides formed while discussing this puzzle with only a handful of total people talking about it.