In families with 3 children, a boy and a girl are equally likely at each birth. $X$ is the number of girls. What is $P(X \ge 2)$?

Explanation

1. 01

   $P(X = 2) = \frac{3}{8}$ (GGB, GBG, BGG) and $P(X = 3) = \frac{1}{8}$.

2. 02

   $P(X \ge 2) = \frac{3}{8} + \frac{1}{8} = \frac{1}{2}$.