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The heights of people h metres in a community are normally distributed with mean µ and standard deviation σ. It is given that P(h < 1.2) = 0.15 and P(h > 1.6) = 0.10. Calculate the mean µ and standard deviation σ of the heights of the people.

Mathematics

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Answers

4 parts
a

1.379 m\text{1.379 m}

0.173 m\text{0.173 m}

b

1.262 m\text{1.262 m}

1.495 m\text{1.495 m}

c

0.233 m\text{0.233 m}

d

0.438\text{0.438}

Step-by-step solution

6 steps

Here's how to solve the problem:

Part a· 2 steps

Calculate the mean μ\mu and standard deviation σ\sigma of the heights.

  1. Convert the given probabilities to standard normal z-scores.

    Given P(h<1.2)=0.15P(h < 1.2) = 0.15. We standardize this to P(Z<1.2−μσ)=0.15P\left(Z < \frac{1.2 - \mu}{\sigma}\right) = 0.15.
    From the standard normal distribution table (or calculator), the z-score corresponding to a cumulative probability of 0.15 is z1≈−1.0364z_1 \approx -1.0364.
    So, we have the equation:

    1.2−μσ=−1.0364(1)\frac{1.2 - \mu}{\sigma} = -1.0364 \quad (1)

    Given P(h>1.6)=0.10P(h > 1.6) = 0.10. This means P(h<1.6)=1−0.10=0.90P(h < 1.6) = 1 - 0.10 = 0.90. We standardize this to P(Z<1.6−μσ)=0.90P\left(Z < \frac{1.6 - \mu}{\sigma}\right) = 0.90.
    From the standard normal distribution table, the z-score corresponding to a cumulative probability of 0.90 is z2≈1.2816z_2 \approx 1.2816.
    So, we have the equation:

    1.6−μσ=1.2816(2)\frac{1.6 - \mu}{\sigma} = 1.2816 \quad (2)
  2. Solve the system of linear equations for μ\mu and σ\sigma.

    From (1): 1.2−μ=−1.0364σ1.2 - \mu = -1.0364\sigma
    From (2): 1.6−μ=1.2816σ1.6 - \mu = 1.2816\sigma
    Subtract equation (1) from equation (2):

    (1.6−μ)−(1.2−μ)=1.2816σ−(−1.0364σ)(1.6 - \mu) - (1.2 - \mu) = 1.2816\sigma - (-1.0364\sigma) 0.4=(1.2816+1.0364)σ0.4 = (1.2816 + 1.0364)\sigma 0.4=2.3180σ0.4 = 2.3180\sigma σ=0.42.3180≈0.17256\sigma = \frac{0.4}{2.3180} \approx 0.17256

    Substitute the value of σ\sigma back into the first equation:

    1.2−μ=−1.0364×0.172561.2 - \mu = -1.0364 \times 0.17256 1.2−μ=−0.178771.2 - \mu = -0.17877 μ=1.2+0.17877\mu = 1.2 + 0.17877 μ=1.37877\mu = 1.37877

    Rounding to three decimal places:
    The mean μ\mu is 1.379 m\boxed{\text{1.379 m}}
    The standard deviation σ\sigma is 0.173 m\boxed{\text{0.173 m}}

Part b

1.262 m\text{1.262 m}

Calculate the lower and upper quartiles.

  1. Find the z-scores for the lower and upper quartiles.

    The lower quartile (Q1Q_1) corresponds to P(h<Q1)=0.25P(h < Q_1) = 0.25. The z-score for 0.25 is zQ1≈−0.6745z_{Q_1} \approx -0.6745.
    The upper quartile (Q3Q_3) corresponds to P(h<Q3)=0.75P(h < Q_3) = 0.75. The z-score for 0.75 is zQ3≈0.6745z_{Q_3} \approx 0.6745.

  2. Calculate Q1Q_1 and Q3Q_3 using μ≈1.37877\mu \approx 1.37877 and σ≈0.17256\sigma \approx 0.17256.

    Q1=μ+zQ1σ=1.37877+(−0.6745)(0.17256)Q_1 = \mu + z_{Q_1}\sigma = 1.37877 + (-0.6745)(0.17256) Q1=1.37877−0.11637Q_1 = 1.37877 - 0.11637 Q1=1.2624Q_1 = 1.2624 Q3=μ+zQ3σ=1.37877+(0.6745)(0.17256)Q_3 = \mu + z_{Q_3}\sigma = 1.37877 + (0.6745)(0.17256) Q3=1.37877+0.11637Q_3 = 1.37877 + 0.11637 Q3=1.49514Q_3 = 1.49514

    Rounding to three decimal places:
    The lower quartile is 1.262 m\boxed{\text{1.262 m}}
    The upper quartile is 1.495 m\boxed{\text{1.495 m}}

Part c

0.233 m\text{0.233 m}

Calculate the inter-quartile range.

The inter-quartile range (IQR) is the difference between the upper and lower quartiles.

IQR=Q3−Q1\text{IQR} = Q_3 - Q_1 IQR=1.49514−1.2624\text{IQR} = 1.49514 - 1.2624 IQR=0.23274\text{IQR} = 0.23274

Rounding to three decimal places:
The inter-quartile range is 0.233 m\boxed{\text{0.233 m}}

Part d

0.438\text{0.438}

Calculate P(∣h−μ∣<0.1)P(|h - \mu| < 0.1).

  1. Rewrite the inequality and standardize.

    The inequality ∣h−μ∣<0.1|h - \mu| < 0.1 means −0.1<h−μ<0.1-0.1 < h - \mu < 0.1.
    Divide by σ\sigma:

    −0.1σ<h−μσ<0.1σ\frac{-0.1}{\sigma} < \frac{h - \mu}{\sigma} < \frac{0.1}{\sigma}

    Using σ≈0.17256\sigma \approx 0.17256:

    −0.10.17256<Z<0.10.17256\frac{-0.1}{0.17256} < Z < \frac{0.1}{0.17256} −0.5795<Z<0.5795-0.5795 < Z < 0.5795
  2. Calculate the probability.

    P(−0.5795<Z<0.5795)=P(Z<0.5795)−P(Z<−0.5795)P(-0.5795 < Z < 0.5795) = P(Z < 0.5795) - P(Z < -0.5795)

    Since the normal distribution is symmetric, P(Z<−z)=1−P(Z<z)P(Z < -z) = 1 - P(Z < z).

    P(−0.5795<Z<0.5795)=P(Z<0.5795)−(1−P(Z<0.5795))P(-0.5795 < Z < 0.5795) = P(Z < 0.5795) - (1 - P(Z < 0.5795)) P(−0.5795<Z<0.5795)=2×P(Z<0.5795)−1P(-0.5795 < Z < 0.5795) = 2 \times P(Z < 0.5795) - 1

    From the standard normal distribution table, P(Z<0.5795)≈0.7189P(Z < 0.5795) \approx 0.7189.

    P(∣h−μ∣<0.1)=2×0.7189−1P(|h - \mu| < 0.1) = 2 \times 0.7189 - 1 P(∣h−μ∣<0.1)=1.4378−1P(|h - \mu| < 0.1) = 1.4378 - 1 P(∣h−μ∣<0.1)=0.4378P(|h - \mu| < 0.1) = 0.4378

    Rounding to three decimal places:
    P(∣h−μ∣<0.1)P(|h - \mu| < 0.1) is 0.438\boxed{\text{0.438}}

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