August 2012 Brain Teaser Solution

Q: A group of four people has to cross a bridge at night. A flashlight is needed to light the path and they only have one flashlight. No more than two people can cross the bridge simultaneously, and it takes different time for the people in the group to cross the bridge: Abby crosses the bridge in 1 minute, Bob crosses the bridge in 2 minutes, Cathy crosses the bridge in 5 minutes, David crosses the bridge in 10 minutes. How can the group cross the bridge in 17 minutes?

A: Abby and Bob go across:    2 minutes
Bob returns with flash:     2 minutes
Cathy and David go across:   10 minutes
                             ----------
                             14 minutes

Now Abby, Cathy and David have crossed, so one of these needs to return 
with the flashlight and afterwards go across with Bob in 3 minutes:

With many thanks to:
http://www.math.utah.edu/~cherk/puzzles.html