2007-06-22

Crossing a Bridge

Four men need to cross a bridge at night:
- No more than two can cross at a time
- They need a flashlight to cross, and they have only one
- They all start on the west side and all need to end up on the east side
- They take 10, 5, 2, and 1 minutes to cross, respectively

What's the most efficient way for them to cross?

And to add a note of suspense: a madman turns up after 18 minutes to kill anyone left on the west side or still on the bridge (i.e., 19 minutes is not the correct answer!).

Thanks go to Darren—this tricky one is from him.

6 comments:

Zootenany Hoodlum said...

You know I think I know this one. This is a Dad one - he broke my brain with it when I was short. I remember the broken brain but not, immediately, the solution. I have an idea but not one that I will share, in an effort to not ruin it for those who have not before heard it.

PS I mentioned praying in my blog. I felt your pain.

Darren said...

MWAHAHAHAHAHAHA!!!!!!

Mark Hanington said...

Given the 18 minute timeline, if I were one of these guys I'd say we set an ambush up and kill the madman by clubbing him to death with the flashlight. Then we can either take 19 minutes or wait until daylight and do it in 12. (10 and 5 go, then 2 and 1)

fiona-h said...

mark - I like the way you think! bonus points for the creativity and bloodshed.

Molly said...

AHAH! I wasted far too much time on this but finally I've got them all across in 17 minutes. I like Mark's solution better, though.

fiona-h said...

Oh hooray, Molly!! Well done.
I spent a couple of days on it and could only manage to solve it with a hint from Darren. (Mr. Kite quietly solved it over email.)