Given 20 people, how many ways are there to form a committee containing at least three people?

$\displaystyle \sum_{k=3}^{20}\left(\frac{20}{k}\right)$ is one answer but a shorter calculation would be $\displaystyle 2^{20}-\left({20\atop 1}\right)-\left({20\atop 2}\right)$.

Does this look correct?