conditional probability? coloured marbles - one is revealed

[graham]

2 white marbles, 1 black marble and one yellow in a bag.

Two are drawn.

One is later revealed to be white.

What is the probability that the other is also white?

 

[stapel]

2 white marbles, 1 black marble and one yellow in a bag.

Two are drawn.

One is later revealed to be white.

What is the probability that the other is also white?

Are the two drawn at the same time, or is the first put back in before the second is drawn?

When you reply, please include a clear statement of your thoughts and efforts so far. Thank you!