Ok, I think i finally understood it now. I made a diagramm of it:



As I found out when making the C++ program, I found out that I can make a "normal" tree to see the overall propabilities and rule out those that do not meet the requirements. Then I can divide all other propabilities by the remaining propability which results to 33.3%.

But I wonder if this is valid. Because I still think the information that the child that is a boy is the older one shouldn't make any difference. But Damocles teacher said it and he's got authoritah...


EDIT: For some reason you can't see the picture in my post. At least I can't. But this link works: http://www.brogames.de/TwoBoys.jpg