CROSSING THE BRIDGE AT NIGHT

A group of four DeWitt Middle School students wish to cross a bridge without siderails in the middle of the jungle very late at night. They have one lantern to share. No one may take a step without holding the lantern (it's dark at night with no moon). No more than two people may be on the bridge at one time, not even for a moment. What is the shortest time it would take for all of them to get across? Use the following information to solve the problem:

The first student, Joe, can cross the bridge alone in 5 minutes. (He's a track star.)
The second student, Torey, can cross the bridge alone in 10 minutes.
The third student, Sarah, can cross the bridge alone in 20 minutes.
The fourth student, Mike, can cross the bridge alone in 25 minutes. (He has a sore ankle).

Now, when two students walk together, they must move at the rate of the slowest person!

For the answer (try not to look right away!).

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