Maria Goeppert was a German physicist. She received the Nobel Prize in Physics in 1963 for the discovery of nuclear orbital structure. She graduated in 1930 using in her thesis the calculus of probabilities to analyse the orbit of the electron. Then solve the following problem:

Let's suppose that the probability of being born a boy or a girl is the same. 

What is the probability that, in a family of 4 daughters and sons, exactly two are girls?