The Bridge Crossing Puzzle - Solved

There are four people. An old professor, his wife and two young students, Alice and Bob. Professor takes 8 minutes to cross the bridge. wife takes 5 minutes to cross the bridge. Alice takes two minutes and Bob takes one minute to cross the bridge. They have 15 minutes in total. At a time maximum, two people can cross the bridge. There should be torch available with the people while crossing the bridge. How will they reach in 15 minutes?