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.

Re 1/4:

half the time, there is a black in the bag

half that time, a white will ne pulled

so 1/4

?????????????