If a patient doesn't receive treatment for a certain injury within 2 hours after injury, the patient's chance of survival starts decreasing by 15% each hour. If no treatment has been given within 4 hours after injury, roughly, what chance of survival will the patient have?
At the end of two hours, the patient still has a 100% chance of survival. After three hours, he has an 85% of survival, or 0.85. After four hours, he has an 85% of the 85% he had at the end of three hours: 0.85 * 0.85 = 72.25% chance of survival.