Answer:
1 hour and 20 minutes
Explanation:
If 4 people take 3 hours, 1 person will take 4 times as long. This is 4*3=12 hours. If 9 people do it, it will take 1/9 of the time 1 person takes to do it, which is 12/9 = 1.3 recurring, or 1 and 1/3 of an hour.