Answer:
11:45 AM
Explanation:
We have that in one hour the body temperature decreased 
went down in 
81.7 - 80.2 = 1.2 
 i.e. 1.2 degrees. 
Normal body temperature is 98.6 degrees. 
The difference between normal and 11 pm is 98.6-81.7 = 16.9 
i.e. 16.9 degrees 
now to know how much it decreases every hour: 
16.9/ 1.5 = 11.26, that is 11 hours and 15 minutes (1/4 of an hour) 
If that was 11:00 PM - 11:15 minutes, the murder was at 11:45 AM