Determining the Probability using Mean and Standard Deviation

The weights of adult Americans are normal with a mean of 164 lbs. and a standard deviation of 24 lbs. Two random samples are taken with 10 adult Americans in each sample.

(a) What is the probability that both samples have a mean greater than 180 lbs.?

(b) What is the probability that, when the two samples are put together, the mean weight of all 20 people will be greater than 180 lbs?

(c) Compare the results in (a) and (b). Explain.

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...7.5) = P(z>2.1) = (from z-table) 0.0179
P(Xbar1>180)= P(Xbar2>180) = 0.0179
P (Xbar1>180) and (Xbar2>180) = P(Xbar1>180) * P(Xbar2>180) =
= ...