If the man committed suicide, he would've left the window open and you wouldn't have had to open it.
When the professor confirmed Johnny's anonymity (the professor didn't know who he was), Johnny quickly slipped his exam paper into the pile of exams and ran off, so the professor wouldn't know which exam paper deserved an F. Congratulations on sticking through the whole riddle.
The recording played a gunshot inside it. If the man committed suicide, he wouldn't have been able to stop the recording after he pulled the trigger.
5 minutes. We can assume that one chicken is placed in each microwave and that each chicken takes 5 minutes to cook.
47 days. Whatever day the lily pads covered half the pond, the whole pond would be covered the next day.