Consider families with two children. assuming boys and girls are equally likely, find the probability that both children are boys given that at least one of them is a boy.

So, there are 2 children. There is a 50/50 chance there will be one boy born, but then the text said that it was given. That means we only have to look at one child. Since we only have to measure one child, and there is a 50/50 chance of a boy or a girl, there is a 50% chance there will be a total of 2 boys.