In a bag, there is either a black or white marble: equal probability.
Without looking, you insert a white marble into this bag.
Then you randomly pick a marble from this bag and it turns out to be white.
What is the probability that the marble left in the bag is black?
I'm stuck: either 1/3 or 1/4; what is it and why? Thanks.
half the time, there is a black in the bag
half that time, a white will ne pulled