Jilly, Zach, Malia and Jerimy wants to cross a weak bridge. The bridge can only carry 3 people as they only have a torch and 3 of them have to walk together.
The following are the time needed to cross the bridge for each person:
Jilly: 20 minutes
Zach: 10 minutes
Malia: 5 minutes
Jerimy: 1 minutes
What is the fastest time that all of them can cross the bridge?
Note: The time required for three people to cross a bridge is equal to the time required by the slowest person