Question

Suppose that replacement times for washing machines are normally distributed with a mean of 9.3 years and a standard deviation of 1.1 years. Find the probability that 70 randomly selected washing machines will have a mean replacement time less than 9.1 years. Your answer should be a decimal rounded to the fourth decimal place.

Answer 1

The probability is 0.0643

Explanation:

* Lets revise some definition to solve the problem

- The standard deviation of the distribution of sample means is called σM

- σM = σ/√n , where σ is the standard deviation and n is the sample size

- z-score = (M - μ)/σM, where M is the mean of the sample , μ is the mean

of the population

* Lets solve the problem

- The mean of the washing machine is 9.3 years

∴ μ = 9.3

- The standard deviation is 1.1 years

∴ σ = 1.1

- There are 70 washing machines randomly selected

∴ n = 70

- The mean replacement time less than 9.1 years

∴ M = 9.1

- Lets calculate z-score

∵ σM = σ/√n

∴ σM = 1.1/√70 = 0.1315

∵ z-score = (M - μ)/σM

∴ z-score = (9.1 - 9.3)/0.1315 = - 1.5209

- Use the normal distribution table of z to find P(z < -1.5209)

∴ P(z < -1.5209) = 0.06426

∵ P(M < 9.1) = P(z < -1.5209)

∴ P(M < 9.1) = 0.0643

* The probability is 0.0643