The random variable X is normally distributed with mean μ and standard deviation σ.
- Given that 5σ = 3μ, find P(X < 2μ).
- With a different relationship between μ and σ, it is given that P(X < 1/3μ) = 0.8524. Express μ in terms of σ.
Solution
(i) To find P(X < 2μ), we standardize the variable:
\(P(X < 2μ) = P\left( z < \frac{2μ - μ}{σ} \right) = P\left( z < \frac{μ}{σ} \right).\)
Given 5σ = 3μ, we have \(\frac{μ}{σ} = \frac{5}{3}\).
\(Thus, P(X < 2μ) = P\left( z < \frac{5}{3} \right) = 0.952.\)
(ii) We are given P(X < \(\frac{1}{3}μ\)) = 0.8524. Standardizing gives:
\(P\left( z < \frac{\frac{1}{3}μ - μ}{σ} \right) = P\left( z < \frac{-2μ}{3σ} \right).\)
From the standard normal distribution table, \(\frac{-2μ}{3σ} = -1.047\).
Solving for μ, we get \(\frac{-2μ}{3σ} = -1.047\) which gives \(μ = -1.57σ\).
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