If the selected ball is white, what is the probability that the bag originally contained a white ball?

Question

We are supposed to use tree diagrams to solve. 

 

A bag is equally likely to contain either one white ball or one red ball. A white ball is added to the bag, and then a ball is selected at random from the bag. If the selected ball is white, what is the probability that the bag originally contained a white ball?

Answer 1

 

The probability that the bag originally contained a white ball is 50%.

 

We were given that it contained 1 white or 1 red.  Adding a white ball to the bag, and then picking a white ball, does not change the original probability prior to that event.