Binomial distribution
Tossing a coin $n$ times, how likely to get $k$ heads?
$$
p(k) =
\begin{cases}
{n \choose k}p^k(1-p)^{n-k}, & \text{if $k = 0, 1, \cdots, n$,} \\
0, & \text{otherwise.}
\end{cases}
\\[16pt]
\text{where $p$ is the probability the coin lands on heads.}
$$