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Question:

A person who has a heart attack can be saved 90% of the time if a defibrillator is used within 5 minutes of the heart attack. Of the people that have heart attacks, defibrillators are within the vicinity 40% of the time; however, they are only used 60% of the time even when they are in the surrounding area. How many people who suffer a heart attack are saved because of the use of a defibrillator?

A 22%
explanation

If a defibrillator is used, we know that the patient will have a 90% success rate; however, only 40% of the people have this option available. Therefore we take .9 *.4 and know that only 36% of the people have this opportunity.
However, we also know that of the 36% of suffers, only 60% of the time is a defibrillator used. Therefore we must take .6 * .36 to find out how many patients are saved by the use of a defibrillator. This results in .216 or 22% of patients are saved by the use of a defibrillator.