asorbus
I assume that the sample space is: [(b,b),(b,g),(g,b),(g,g)] where the four possible birth sequences are [(younger child is boy, older child is boy), (younger child is boy, older child is girl), (younger child is girl, older child is boy), (younger child is girl, older child is girl)] and the sequences are all equally likely.
So now if you ask, what is the probability that the person has a girl and a boy given that at least one of them is a girl, you can discard (b,b) from your sample space because you know that there is at least one girl.
Now your sample space is [(b,g),(g,b),(g,g)] and the odds that the person has a girl and a boy are 2/3.
It's hard to stay alert as a young kid during such a long school day, with class times being so long. When I transferred to public school, I met a lot of very intelligent people my age, and I was surprised to see that. For the first year after transferring, I felt that I was too smart to be there, since I had just transferred from a charter school. It turned out that I had met people who were much smarter than me and just as insightful.
Sure American kids don't do so well on international tests, but maybe that's because they're not taught to take tests, they're taught to learn and apply what they learn. I don't know about you, but I'd rather my kids grow up knowing how to learn new things and broaden their horizons, rather than grow up knowing how to write answers down on a piece of paper.