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The random variable $X \sim N(\mu, 5^2)$ and $P(X < 23) = 0.9192$ (a) Find the value of $\mu$ - Edexcel - A-Level Maths Statistics - Question 2 - 2011 - Paper 2

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The-random-variable-$X-\sim-N(\mu,-5^2)$-and-$P(X-<-23)-=-0.9192$--(a)-Find-the-value-of-$\mu$-Edexcel-A-Level Maths Statistics-Question 2-2011-Paper 2.png

The random variable $X \sim N(\mu, 5^2)$ and $P(X < 23) = 0.9192$ (a) Find the value of $\mu$. (b) Write down the value of $P(\mu < X < 23)$.

Worked Solution & Example Answer:The random variable $X \sim N(\mu, 5^2)$ and $P(X < 23) = 0.9192$ (a) Find the value of $\mu$ - Edexcel - A-Level Maths Statistics - Question 2 - 2011 - Paper 2

Step 1

(a) Find the value of μ.

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Answer

To find the value of μ\mu, we first standardize the given variable using the formula:

Z=XμσZ = \frac{X - \mu}{\sigma}

In this case, σ=5\sigma = 5 and we know that:

P(X<23)=0.9192P(X < 23) = 0.9192

We can standardize this:

P(Z<23μ5)=0.9192P\left(Z < \frac{23 - \mu}{5}\right) = 0.9192

From the standard normal distribution table, a probability of 0.9192 corresponds to a Z-value of approximately 1.40. Therefore, we set:

23μ5=1.40\frac{23 - \mu}{5} = 1.40

Now, solving for μ\mu:

23μ=1.40×523 - \mu = 1.40 \times 5

23μ=723 - \mu = 7

μ=237=16\mu = 23 - 7 = 16

Step 2

(b) Write down the value of P(μ < X < 23).

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Answer

Since we have found μ=16\mu = 16, we need to calculate:

P(16<X<23)P(16 < X < 23)

This is equivalent to:

P(X<23)P(X<μ)=P(X<23)P(X<16)P(X < 23) - P(X < \mu) = P(X < 23) - P(X < 16)

Using the properties of the normal distribution:

  1. We know that P(X<23)=0.9192P(X < 23) = 0.9192.
  2. From the standard normal distribution, we can find that P(X<16)P(X < 16) corresponds to a Z-value of -1.0.

Thus, P(X<16)=0.1587P(X < 16) = 0.1587. Therefore:

P(16<X<23)=0.91920.1587=0.7605.P(16 < X < 23) = 0.9192 - 0.1587 = 0.7605.

In conclusion, the answer is:

P(16<X<23)=0.7605P(16 < X < 23) = 0.7605.

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