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.
A) 0.4357
B) 0.0714
C) 0.0643
D) 0.4286

Question

Quizplus · Accepted Answer

The standard error of the mean is 1.1/sqrt(70). The z-score for a mean of 9.1 is (9.1 - 9.3) / (1.1/sqrt(70)). Using the z-score, we find the probability from the standard normal distribution table.

Suppose That Replacement Times for Washing Machines Are Normally Distributed