# Thread: Marbles in a jar (probability)

1. ## Marbles in a jar (probability)

A jar contains a marble, it may is black or white with the same probability.
We put a white marble in the jar and then we draw a marble. Suppose that the marble we draw is white. What is the probability the first draw to be black?

Facts:
W: the first marble is white, B: the first marble is black.
DW: we draw a white marble, DB: we draw a black marble.

I think the answer is 1/2 but how can i proove it ? Can anybody help me ?

2. Originally Posted by Anglela
A jar contains a marble, it may is black or white with the same probability.
We put a white marble in the jar and then we draw a marble. Suppose that the marble we draw is white. What is the probability the first draw to be black?

Facts:
W: the first marble is white, B: the first marble is black.
DW: we draw a white marble, DB: we draw a black marble.

I think the answer is 1/2 but how can i proove it ? Can anybody help me ?
Draw a tree diagram. It will make the following answer (based on conditional probability) easier to understand: $\displaystyle \frac{\frac{1}{4}}{\frac{1}{4} + \frac{1}{2}} = \frac{1}{3}$.