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Exercises · Q7

Q.If X∼N(500,64)X \sim N(500, 64), find P(X>510)P(X > 510).

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✓ Free question

Step 1 — Identify parameters. μ=500\mu=500, variance =64⇒σ=64=8=64 \Rightarrow \sigma=\sqrt{64}=8.

Step 2 — Standardize.

z=510−5008=108=1.25z = \dfrac{510-500}{8} = \dfrac{10}{8} = 1.25

Step 3 — Apply the complement rule.

P(X>510)=1−Φ(1.25)=1−0.8944=0.1056P(X>510) = 1-\Phi(1.25) = 1-0.8944 = 0.1056

Independent second method (dual-solve check): re-derive using symmetry — P(X>510)P(X>510) with z=1.25z=1.25 should equal P(X<490)P(X<490) with z=−1.25z=-1.25 (the mirror value below the mean), since both are equally far from μ=500\mu=500. Φ(−1.25)=1−Φ(1.25)=1−0.8944=0.1056\Phi(-1.25)=1-\Phi(1.25)=1-0.8944=0.1056 — identical to the direct answer, confirming correctness via the curve's symmetry.

✓Final answer

P(X>510)=0.1056P(X>510) = 0.1056, i.e. 10.56%.

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