Before the draw, the bag was white and white or white and black, equally likely. Drawing white is certain from the first but only a half from the second, so the first is now twice as likely. Two chances in three the one left is white.
A bag holds one counter, which is either white or black. You drop in a white counter, shake the bag and draw one out. It’s white. What’s the chance the counter left in the bag is white?
Give me a hintThe hint
Was the white one you drew the one you put in, or the one that was there before?