## Elementary Statistics (12th Edition)

We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$. The probability of having a girl is $\frac{1}{2}$, therefore in 400 births, the number of girls should be approximately 200. Of course, there are many opportunities around 200, therefore it is unlikely to have exactly 380 girls. Also, it is very unusual because it is quite far from 200.