3 students are chosen at random from 5 boys and 4 girls. What is the probability that all 3 are girls?

Explanation

1. 01

   Total ways: ${}^{9}C_3 = 84$. All girls: ${}^{4}C_3 = 4$.

2. 02

   $\frac{4}{84} = \frac{1}{21}$.