Can anyone help me with this puzzle or riddle, whatever you want to call it.

Ok then here goes;

A group of high 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 ( its dark with no moon). No more than 2 people may be on the bridge at one time, not evenfor a moment. What is the shortest time it would take for ll of them to get across?

Info:
Joe can cross bridge alone in 5 minutes
Torey can cross bridge alone in 10 minutes
Sarah can cross bridge alone in 20 minutes
Mike can cross the bridge in 25 minutes due to his sore ankle

Also, when 2 students walk together, the must move at the rate of the slowest person!

First correct answer gets 10 pts!
>.> this has so many combos, i keep doing it but i still dont get it..