I am sorry Marion , but I cannot accept your reasoning on this, and wull have to ask you to refer me to an academic text which explains this and which agrees with your reasoning. I mean you didn't make this problem up yourself, did you?
This problem seems to be related to one which was first aired in Scientific American in the 60's, and which stirred a fair amount of debate. There are three cards, one is red on both sides, one is white on both sides, and one is red on one side and white on the other. The dealer places them in a bag, shakes them then slides out one card onto the table so that only one face is visible. The face is red. What is the probability that the other face is red?