Challenge of the Week

TO MY STUDENTS: When you visit this webpage, email me with your solution 
and, if you are correct, I'll give you a Bonus Point.  (Bonus Points are a 
way to improve your grade.)  New Challenge of the Week questions are usually 
posted on Monday and I'll accept solutions through Thursday.  I'll post the 
correct solution the following weekend.

CHALLENGE OF THE WEEK: December 14 - 18
Christmas Dilemma:
Four of Santa’s elves, Itsy, Binky, Bernard and Moe, are stranded on a patch 
of ice surrounded by water.  The only escape is by way of a treacherous ice 
bridge.  The elves have only one lantern, they are extremely tired from 
making lots of toys and the time is 9:00 PM.  To cross the bridge on his own 
Itsy takes 5 minutes.  Binky (being older and more tired) takes 10 minutes.  
Bernard and Moe are in pretty sad shape and take 20 and 25 minutes, 
respectively.  The bridge will support at most two elves at a time and they 
must carry the lantern while crossing in order to keep from taking a wrong 
step and falling into the ice-cold water.  How can all four elves get across 
the bridge by 10:00 PM so they can still have time to pack Santa’s Sleigh?