The times to pop a 3.4-ounce bag of microwave popcorn without burning it are Normally distributed with a mean time of 140 seconds and a standard deviation of 20 seconds. A random sample of four bags is selected and the mean time to pop the bags is recorded. Which of the following describes the sampling distribution of all possible samples of size four?

The sampling distribution of the sample mean has the same mean as the population, so its mean is 140 seconds. Its standard deviation, called the standard error, is 20/√4 = 10 seconds. Because the individual popping times are Normally distributed, the sample means are also Normally distributed. Thus, choice D, approximately Normal with a mean of 140 seconds and a standard deviation of 10 seconds, is correct.

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