Three Heads in Four Flips

Count sequences with exactly three heads.

Use $\left(\genfrac{}{}{0ex}{}{4}{3}\right)$.

1. 1. Count

   $$\frac{\left(\genfrac{}{}{0ex}{}{4}{3}\right)}{2^4}=\frac{4}{16}=\frac{1}{4}$$

2. 2. Key idea

   Binomial probability: $\left(\genfrac{}{}{0ex}{}{n}{k}\right)p^k(1-p{)}^{n-k}$.

Answer: 1/4