Four boy scouts wanted to determine the least amount time that they could cross a narrow suspended footbridge over a ravine.  During the day each boy was timed. 

Scout A took 5 minutes. 

Scout B took 2 minutes.

Scout C took only 1 minute to cross. 

Scout D took 10 minutes to get across.

Now, they plan to cross at night, but there are several problems.  The bridge can hold two people at most and they know that they’ll have to carry a flashlight.   But, the flashlight must be carried back across the bridge.  It cannot be thrown, as the distance is too great.  The scouts figure that the best they can do is make it in 17 minutes.