How to get audience audio in a small conference/training for 40-80 people? What is the best way to record/capture audience audio when questions asked or interacted with the presenter without passing secondary mic hand to hand? 

Currently we have a presenter's mic that is going through audio board and on camera mic that captures general audience audio. But it creates a bad audio as camera is placed at the back at the room. 

Has any one tried placing mics around the room to capture it? Any solutions would be greatly appreciated.
Here's a video that covers everything you need to know about sound recording for video: http://nofilmschool.com/2015/08/24-sound-basics-will-help-you-lay-sturdy-foundation

If you don't have 18 minutes to spend, the specific answer to your question is that basically you cannot capture well the human voice when a mic is more than 3 feet away.  Even if you use a high-quality mic and a high-quality pre-amp, the quality of your audio will drop precipitously as you break the 3-foot rule.  (There are exceptions such as professional singers and actors hitting PZM stage mics, but even that is not great.)  

If you place mics around the room in such a way that nobody has to throw their voice more than 3 feet to reach a microphone, then you might have a solution.  But that might require 20-40 microphones, and an audio system capable of recording 20-40 inputs simultaneously.  Impractical.

Alternatively, you could have 3 mics, one for each zone of 15-30 people, which you can rapidly run to whichever audience member is trying to speak.  When they hold the mic (at least 1-2" from their mouth and at most 3 feet away), you should get fairly good results.  That would require an audio system that can record four tracks simultaneously (one for the podium mic and 3 for the audience mics), which is very simple nowadays.