Midterm 2A SP10 Solutions

# The mean of a random variable x can never be negative

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17. The mean of a random variable X can never be negative. a. True b. False 18. Which of the following is not true about a continuous random variable? 19. If two events are independent, what is the probability that they both occur? 20. Suppose the standard deviation of X equals 4. Then the standard deviation of 2X is equal to which of the following? a.4 b. 2 c.8 d. 16 21. If A and B are disjoint events where P(A) = 0.20 and P(B) = 0.30, then P(A and B) equals: 22. If P(A) = 0.84, P(B) =0.76 and P(A or B) =0.90, then P(A and B) is: a. 0.64 b. 0.70 c. 0.14 d. Can’t tell from the information given. 5

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Problems 23-24. Suppose 63% of the U.S. population is annoyed by other people’s cell phone conversations. Of those who are annoyed, only 32% have cell phones. Of those not annoyed, 51% own cell phones. 23. What is the chance that a person in the U.S. has a cell phone? a.16% b. 39% c.32% d. 83% 24. Choose 10 people from the U.S. at random. What is the chance that at least one of them is annoyed by other people’s cell phone conversations? 25. It is possible for a relationship that you found between two variables to actually reverse direction if you add a third confounding variable to the mix. 26. Which of the following CANNOT be used to test for independence? 27. Suppose X is a random variable whose only possible value is 10. Then µ x = 10. a.TRUE b. FALSE 28. Suppose X is a random variable whose only possible value is 10. Then σ x = 10. 29. Suppose you have a deck of 52 cards and you draw one at random. Let A be the event the card is a 2, and let B be the event that the card is red. The events A and B in this situation are independent. a.YES b. NO
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