Bags of Marbles

 
Bags of Marbles

Problem statement:

You have three bags, each containing two marbles. Bag A contains two white marbles, Bag B contains two black marbles, and Bag C contains one white marble and one black marble.

You pick a random bag and take out one marble.

It is a white marble.

What is the probability that the remaining marble from the same bag is also white?

Done thinking? Click here to view the solution.

2/3 (not 1/2)

You know that you do not have Bag B (two black marbles) so there are three possibilities:
 
You chose Bag A, first white marble. The other marble will be white.
You chose Bag A, second white marble. The other marble will be white.
You chose Bag C, the white marble. The other marble will be black.
 
So 2 out of 3 possibilities are white.

Why not 1/2? You are selecting marbles, not bags.