## Essentials of Statistics (5th Edition)

$0.5$
We know that $probability=\frac{number \ of \ good \ outcomes}{number\ of\ all\ outcomes}$. When having 4 kids, there are 4 opportunities:boy-boy, boy-girl, girl-boy, girl-girl. Therefore the probability of getting one child of each sex is (2 opportunities out of 4):$\frac{2}{4}=0.5$