Markov's inequality

Theorem

For any random variable $X$ which only takes non-negative values, and any positive $t$ ,

\mathrm{Pr}[X \geq t]\leq \frac{\mathbb{E}[X]}{t}

equivalently,

\mathrm{Pr}[X \geq \alpha\cdot\mathbb{E}[X]]\leq\frac{1}{\alpha}