Public Health and Nutrition Emergency defibrillators are now located in many public buildings. However, the U.S. Food and Drug Administration (FDA) is concerned about the reliability of these devices. Approximately 45,000 devices failed during the past seven years.41 Suppose the FDA claims that the probability of an emergency defibrillator working correctly is 0.90, and 30 of these devices are selected at random and tested.

a. Find the probability that exactly 28 devices will work correctly.

b. Find the probability that at least 25 devices will work correctly.

c. Suppose only 20 of the devices work correctly. Is there any evidence to suggest that the proportion of emergency defibrillators that work correctly has changed? Justify your answer.