During a flight on an airplane, Eric strikes up a chat with Erica, the person sitting next to him. It turns out that Erica has two kids, and at least one of them is a girl born on a Tuesday. Being a mathematician, Eric decides to find the probability that Erica has a boy and a girl before asking her. What is the probability that Erica has one boy, and one girl? Assume an equal chance of giving birth to either gender and an equal chance to giving birth on any day.
I was looking at the solution to this question and for some reason they were looking at the number of pairs of $bg$ in a two week time period. Why not a week period?