MHB James' question about Normal Distribution

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The discussion focuses on calculating probabilities related to a normal distribution with a mean (μ) of 5 and a standard deviation (σ) of 2. For part (a), it confirms that Pr(X < 3) corresponds to Pr(Z < -1), resulting in a value of a = -1. In part (b), it finds that Pr(X > 8) translates to Pr(Z > 1.5), leading to b = 1.5. Lastly, part (c) establishes that Pr(X > 6) equals Pr(Z < -0.5), giving c = -0.5. The calculations are validated through a clear derivation process, emphasizing the relationship between X and Z.
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(a) We are told $\displaystyle \begin{align*} \textrm{Pr}\,\left( X < 3 \right) = \textrm{Pr}\,\left( Z < a \right) \end{align*}$, so if $\displaystyle \begin{align*} x = 3 \end{align*}$ and $\displaystyle \begin{align*} z = a \end{align*}$ then we have

$\displaystyle \begin{align*} z &= \frac{x - \mu}{\sigma} \\ a &= \frac{3 - 5}{2} \\ a &= \frac{-2}{\phantom{-}2} \\ a &= -1 \end{align*}$(b) We are told $\displaystyle \begin{align*} \textrm{Pr}\,\left( X > 8 \right) = \textrm{Pr}\,\left( Z > b \right) \end{align*}$, so if $\displaystyle \begin{align*} x = 8 \end{align*}$ then we have

$\displaystyle \begin{align*} z &= \frac{x - \mu}{\sigma} \\ b &= \frac{8 - 5}{2} \\ b &= \frac{3}{2} \\ b &= 1.5 \end{align*}$(c) We are told $\displaystyle \begin{align*} \textrm{Pr}\,\left( X > 6 \right) = \textrm{Pr}\,\left( Z < c \right) \end{align*}$, so by symmetry, $\displaystyle \begin{align*} \textrm{Pr}\,\left( X > 6 \right) = \textrm{Pr}\,\left( Z > -c \right) \end{align*}$, and thus if $\displaystyle \begin{align*} x = 6 \end{align*}$ then

$\displaystyle \begin{align*} z &= \frac{x - \mu}{\sigma} \\ -c &= \frac{6 - 5}{2} \\ -c &= \frac{1}{2} \\ c &= -\frac{1}{2} \end{align*}$
 
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(a) We are told $\displaystyle \begin{align*} \textrm{Pr}\,\left( X < 3 \right) = \textrm{Pr}\,\left( Z < a \right) \end{align*}$, so if $\displaystyle \begin{align*} x = 3 \end{align*}$ and $\displaystyle \begin{align*} z = a \end{align*}$ then we have

$\displaystyle \begin{align*} z &= \frac{x - \mu}{\sigma} \\ a &= \frac{3 - 5}{2} \\ a &= \frac{-2}{\phantom{-}2} \\ a &= -1 \end{align*}$(b) We are told $\displaystyle \begin{align*} \textrm{Pr}\,\left( X > 8 \right) = \textrm{Pr}\,\left( Z > b \right) \end{align*}$, so if $\displaystyle \begin{align*} x = 8 \end{align*}$ then we have

$\displaystyle \begin{align*} z &= \frac{x - \mu}{\sigma} \\ b &= \frac{8 - 5}{2} \\ b &= \frac{3}{2} \\ b &= 1.5 \end{align*}$(c) We are told $\displaystyle \begin{align*} \textrm{Pr}\,\left( X > 6 \right) = \textrm{Pr}\,\left( Z < c \right) \end{align*}$, so by symmetry, $\displaystyle \begin{align*} \textrm{Pr}\,\left( X > 6 \right) = \textrm{Pr}\,\left( Z > -c \right) \end{align*}$, and thus if $\displaystyle \begin{align*} x = 6 \end{align*}$ then

$\displaystyle \begin{align*} z &= \frac{x - \mu}{\sigma} \\ -c &= \frac{6 - 5}{2} \\ -c &= \frac{1}{2} \\ c &= -\frac{1}{2} \end{align*}$
Your answers look fine but I wouldn't have done things as you did.
You're given that ##\mu = 5## and ##\sigma = 2##, so ##Z = \frac{X - \mu}\sigma##. Substituting for the given parameters, we have ##Z = \frac{X - 5}2 \Rightarrow X = 2Z + 5##.

For part a, ##Pr(X < 3) = Pr(2Z + 5 < 3) = Pr(2z < -2) = Pr(Z < -1)##, so ##a = -1##, same answer that you gave, but probably cleaner in its derivation. The other two parts are done similarly.
 
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