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A patient injury and reduced survival rate as time increases.

If a patient does not receive treatment for a certain injury within 2 hours after the injury, the patient's chance of survival starts decreasing by 15% for each hour.  If no treatment has been given within 4 hours after the injury, roughly, what chance of survival will the patient have?
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1 Answer

According to the problem, the patients survival rate doesn't start to decrease until the first 2 hours are up.  By the end of the third hour, the patients survival rate is 85%.  By the end of the 4th hour, the patients survival rate is now 72.25%.
 
(100%-15%)/100= 0.85
 
Third hour:  100% x (0.85) = 85%
Fourth hour:  85% x (0.85) = 72.25%
 
Or, depending on how you read into the question:
 
Third hour: 100% - 15% = 85%
Fourth hour: 85% - 15% = 70%