: Bridge Riddle : the answer : The fastest way for the four people across is to group similar speeds together. ; Here's the solution : : Adam and Chivon cross with the lantern first. (2 minutes) : Adam crosses back, bringing the lantern (1 minute, 3 mins total) : Julio and Jenn cross, taking the lantern back (10 minuntes, 13 total) : Chivon takes the lantern back to Adam (2 minutes, 15 total) : Chivon and Adam recross, joining their friends (2 minutes, 17 total) : The most common pitfall is to use Adam to ferry the lantern back-and-forth : across the bridge because his travel-time is the shortest. Unfortunately, : this means that Adam crosses the bridge with each person, so we simply : sum up the times (2 + 5 + 10) and add in Adam's two solo trips, giving us : 19 minutes total. Close, but no cigar. : By having Julio and Jenn cross together, you essentially re-coup 5 minutes : of travel time, at the expense of having Chivon and Adam each cross an : extra time (3 minutes), for a net gain of 2 minutes. : © 2001, Aaron Clauset : Disclaimer: This puzzle is not of my devising. It was one posed to me when : I interviewed at Microsoft in the spring of 2001. Took me forever to get the : solution, because I got stuck in the 'common pitfall' explained above. Still, : once I got the solution, it was a great sense of achievement, because I'd been : stuck so long :-)

© Aaron Clauset

 updated 7.24.01