Four persons are standing in front of a canyon. The only
bridge can carry only two persons at once. Because of
the darkness the persons have to use a torch.
Unfortunately there is only one torch. Whenever two
persons have crossed the bridge one person must carry
back the torch. Person A needs 5 min for crossing the
bridge (one way). B needs 10 min, C needs 20 min and
D needs 25 min. Two persons together need the time of
the slower one.
What is the minimum time for the group to cross the bridge?
