If the population doubles every 20 minutes, and the can is full at 6:00PM, then the can must have been half full 20 minutes before 6:00PM or 5:40PM
Likewise the can is:
Full at 6:00PM
Half full at 6:00-0:20 = 5:40PM
1/4 full at 5:40 - 0:20 = 5:20PM
1/8 full at 5:20 - 0:20 = 5:00PM
1/16 full at 5:00 - 0:20 = 4:40PM