“The world is indeed full of peril and in it there are many dark places.”
Brain Tease #2
All the possibilities are Boy-Boy, Boy-Girl, Girl-Boy, Girl-Girl. If one is a boy, you can remove G-G from universe, so the B-B probability is 1 out of 3.
Let's continue
Isildur has four children. His 3 older children are boys, what is the probability that the last one is a boy too?
Answer as a probability, 2 significant figures : 0.XX