P\{|\frac{X_n}{n} - E( \frac{X_n}{n})| \geq \epsilon \} \leq \frac{ Var(\frac{X_n}{n})}{\epsilon^2} \iff P\{|\frac{X_n}{n} - \frac{1}{n}np| \geq \epsilon \} \leq \frac{npq}{n^2\epsilon^2}