0.95 \leq P\left(X \geq 500-n\right) = P\left(500-n \leq X \leq n\right) \approx \Phi\left(\frac{n - \frac{n}{4}}{\frac{\sqrt{3n}}{2}}\right) - \Phi\left(\frac{500-n-\frac{n}{4}}{\frac{\sqrt{3n}}{2}}\right) \approx