: 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 :-)