Math Problem Statement

Assume that women's weights are normally distributed with a mean given by 143 lb and a standard deviation given by 29 lb. (a) If 1 woman is randomly selected, find the probabity that her weight is between 108 lb and 179 lb

(b) If 6 women are randomly selected, find the probability that they have a mean weight between 108 lb and 179 lb

(c) If 95 women are randomly selected, find the probability that they have a mean weight between 108lb and 179 lb

Solution

We are dealing with a normally distributed population of women’s weights with a mean μ=143lb\mu = 143 \, \text{lb} and a standard deviation σ=29lb\sigma = 29 \, \text{lb}. To solve the given questions, we will use Z-scores and the properties of the standard normal distribution.


Part (a):

If 1 woman is randomly selected, find the probability that her weight is between 108 lb and 179 lb.

We convert both bounds (108 lb and 179 lb) into Z-scores using:

Z=XμσZ = \frac{X - \mu}{\sigma}

For 108 lb:

Z1=10814329=35291.21Z_1 = \frac{108 - 143}{29} = \frac{-35}{29} \approx -1.21

For 179 lb:

Z2=17914329=36291.24Z_2 = \frac{179 - 143}{29} = \frac{36}{29} \approx 1.24

Using standard normal distribution tables or a calculator:

  • P(Z<1.21)0.1131P(Z < -1.21) \approx 0.1131
  • P(Z<1.24)0.8925P(Z < 1.24) \approx 0.8925

Now, the probability that the woman's weight is between 108 and 179 lb:

P(108<X<179)=P(Z<1.24)P(Z<1.21)=0.89250.1131=0.7794P(108 < X < 179) = P(Z < 1.24) - P(Z < -1.21) = 0.8925 - 0.1131 = 0.7794

So, the probability is:

0.7794\boxed{0.7794}


Part (b):

If 6 women are randomly selected, find the probability that they have a mean weight between 108 lb and 179 lb.

When dealing with the sample mean, the standard deviation becomes:

σXˉ=σn\sigma_{\bar{X}} = \frac{\sigma}{\sqrt{n}}

Here, n=6n = 6, so:

σXˉ=29611.83\sigma_{\bar{X}} = \frac{29}{\sqrt{6}} \approx 11.83

Now, we recalculate the Z-scores with the new standard deviation:

For 108 lb:

Z1=10814311.832.96Z_1 = \frac{108 - 143}{11.83} \approx -2.96

For 179 lb:

Z2=17914311.833.04Z_2 = \frac{179 - 143}{11.83} \approx 3.04

Using standard normal distribution tables:

  • P(Z<2.96)0.0015P(Z < -2.96) \approx 0.0015
  • P(Z<3.04)0.9988P(Z < 3.04) \approx 0.9988

Now, the probability that the mean weight is between 108 and 179 lb:

P(108<Xˉ<179)=P(Z<3.04)P(Z<2.96)=0.99880.0015=0.9973P(108 < \bar{X} < 179) = P(Z < 3.04) - P(Z < -2.96) = 0.9988 - 0.0015 = 0.9973

So, the probability is:

0.9973\boxed{0.9973}


Part (c):

If 95 women are randomly selected, find the probability that they have a mean weight between 108 lb and 179 lb.

For this larger sample size n=95n = 95, the standard deviation of the sample mean becomes:

σXˉ=29952.97\sigma_{\bar{X}} = \frac{29}{\sqrt{95}} \approx 2.97

Now, we recalculate the Z-scores:

For 108 lb:

Z1=1081432.9711.78Z_1 = \frac{108 - 143}{2.97} \approx -11.78

For 179 lb:

Z2=1791432.9712.12Z_2 = \frac{179 - 143}{2.97} \approx 12.12

Using the standard normal table:

  • P(Z<11.78)P(Z < -11.78) is extremely close to 0.
  • P(Z<12.12)P(Z < 12.12) is extremely close to 1.

Thus, the probability that the mean weight is between 108 and 179 lb:

P(108<Xˉ<179)=P(Z<12.12)P(Z<11.78)10=1P(108 < \bar{X} < 179) = P(Z < 12.12) - P(Z < -11.78) \approx 1 - 0 = 1

So, the probability is:

1.0000\boxed{1.0000}


Summary of Results:

  • (a) Probability for 1 woman: 0.77940.7794
  • (b) Probability for 6 women: 0.99730.9973
  • (c) Probability for 95 women: 1.00001.0000

Do you need further clarification or more details?


Here are 5 related questions:

  1. What is the probability that a randomly selected woman weighs more than 200 lb?
  2. How does increasing the sample size affect the standard deviation of the sample mean?
  3. What is the Z-score corresponding to a weight of 143 lb?
  4. How would the probabilities change if the standard deviation were smaller?
  5. What is the meaning of the central limit theorem in the context of these problems?

Tip: As the sample size increases, the variability of the sample mean decreases, leading to probabilities closer to 1 for values near the population mean.

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Math Problem Analysis

Mathematical Concepts

Normal Distribution
Z-Scores
Probability
Sampling Distribution

Formulas

Z = (X - μ) / σ
σₓ̄ = σ / √n
P(Z < z)

Theorems

Central Limit Theorem

Suitable Grade Level

Grades 11-12 or Introductory College Level