Math Problem Statement

A manufacturer knows that their items have a lengths that are skewed right, with a mean of 14 inches, and standard deviation of 3.6 inches.

If 41 items are chosen at random, what is the probability that their mean length is greater than 15.6 inches?

(Round answer to four decimal places)

Solution

To solve this, we apply the Central Limit Theorem (CLT). Here’s the step-by-step process:


Step 1: Identify key parameters

  • Population mean μ=14\mu = 14
  • Population standard deviation σ=3.6\sigma = 3.6
  • Sample size n=41n = 41
  • Sample mean of interest xˉ=15.6\bar{x} = 15.6

Step 2: Compute the standard error (SE) of the sample mean

The standard error is given by:

SE=σnSE = \frac{\sigma}{\sqrt{n}}

Substitute the values:

SE=3.6413.66.4030.562SE = \frac{3.6}{\sqrt{41}} \approx \frac{3.6}{6.403} \approx 0.562


Step 3: Convert the sample mean to a z-score

The z-score is calculated as:

z=xˉμSEz = \frac{\bar{x} - \mu}{SE}

Substitute the values:

z=15.6140.5621.60.5622.847z = \frac{15.6 - 14}{0.562} \approx \frac{1.6}{0.562} \approx 2.847


Step 4: Find the probability from the z-score

We now find the probability corresponding to a z-score of z=2.847z = 2.847 using the standard normal distribution table or a calculator.

The cumulative probability to the left of z=2.847z = 2.847 is approximately:

P(Z2.847)0.9978P(Z \leq 2.847) \approx 0.9978

Thus, the probability that the mean is greater than 15.6 inches is:

P(xˉ>15.6)=1P(Z2.847)10.9978=0.0022P(\bar{x} > 15.6) = 1 - P(Z \leq 2.847) \approx 1 - 0.9978 = 0.0022


Final Answer:

The probability that the mean length is greater than 15.6 inches is 0.0022 (rounded to four decimal places).


Let me know if you'd like further clarification or more examples! Here are 5 related questions you might find useful:

  1. What is the significance of the Central Limit Theorem in probability calculations?
  2. How does increasing the sample size affect the standard error of the sample mean?
  3. How do you find probabilities when the data is not normally distributed?
  4. What happens if the population standard deviation is unknown?
  5. How do z-scores relate to percentiles in a normal distribution?

Tip: Always check if the sample size is sufficiently large (typically n>30n > 30) for the Central Limit Theorem to apply when working with non-normal populations!

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Math Problem Analysis

Mathematical Concepts

Probability
Central Limit Theorem
Z-scores
Normal Distribution

Formulas

Standard Error (SE) = σ / √n
Z-score = (x̄ - μ) / SE

Theorems

Central Limit Theorem

Suitable Grade Level

Grades 11-12