You choose one of two identical looking bags at random. One bag has three black marbles and one white marble. The other has three white marbles and one black marble.

After choosing a bag you draw one marble out at random. You notice it is black. You then put it back and draw another marble out of the same bag at random.

What is the probability that the second marble drawn is black?

(In reply to re: Have you lost your marbles ? by Brian Smith)

Whoops....Take 2. (Thanks for the correction, Mr. Smith).

Let's say you do this marbleous experiment repeatedly, and draw the initial black marble 200 times. In 100 of those times, on average, you will have a bag with three white marbles and one black marble, and wil pull the second black marble approximately 25 times. In the other 100 times, with the bag with three black marbles and one white marble, you will draw the second black marble 75 times, on average. So out of 200 tries, you will draw the second black marble 25 + 75 times, or 100 times. 50% probability ???? That can't be right....Maybe I should have checked with golfer Shannon Jones before I tried this one..... :-)
Edited on October 13, 2003, 9:47 pm

Posted by Dan on 2003-10-13 21:15:50