## 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 197 girls. On the other hand, it is not unusual because it is quite close to 200.