There are some red balls and some black balls in a bag.
You reach in and grab two balls at random.
The probability that you picked one ball of each color is exactly 0.5.
Which of these numbers is a possible choice for the number of red balls that were originally in the bag?
Clarification: "All of them" and "None of them" refer only to the numerical answers provided, not to each other! (No this isn't a logic question in disguise! ;) )
More probability questions
Image credit: pcchip.deviantart.com