A bag contains red and blue marbles. If you draw two marbles at random without replacement, the probability that both are red is exactly 1/2. What is the smallest possible total number of marbles in the bag?
4
Let r be red marbles and n total marbles. Probability is (r/n) * ((r-1)/(n-1)) = 1/2. For n = 4 and r = 3: (3/4) * (2/3) = 6/12 = 1/2. Thus, 4 is the smallest total number of marbles (3 red, 1 blue).
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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