Five persons have to cross a bridge. The times each needs to cross it are:

   1 min, 3 min, 6 min, 8 min and 12 min.

 It is dark and they only have one lamp, we have the following constraints:

 - the bridge can only be crossed if one has a lamp, while it is too dark.
 - 2 persons at most are allowed to be on the bridge at the same time.
 - the speed of a pair is determined by the speed of the slower person.
 - the lamp can last only 30 min.
 - the lamp cannot be thrown.

 How do they manage to cross the river within 30 min?

 << This is no trick question. >>