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?

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%