Four people need to get from point A to point B. Unfortunately it is pouring down rain and nobody wants to get wet. They have one umbrella between them which can keep two people dry at a time. Person 1 can make the trip from A to B in 1 minute, person 2 in 2 minutes, person 3 in 5 minutes, and person 4 in 8 minutes. Two people travelling together underneath the umbrella will go at the pace of the slower person. How can everyone get from A to B in no more than 15 minutes?
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Person 1 takes person 2 then returns (3 minutes)
Persons 3 and 4 go together and then person 2 comes back with the umbrella (10 minutes)
Persons 1 and 2 then go together (2 minutes; total 15 minutes)