h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Here’s a puzzle I was given yesterday, I thought I’d share it with everyone.

Four people are trying to cross a very fragile bridge in the middle of the night. Only two people are allowed on the bridge at any one time, the bridge is the only way across and you cannot wait until dawn, they will die or something like that. To make matters worse you can only cross with the aid of a torch, and the torch only has enough power for 17 minutes after that nobody will be able to cross or continue to cross, you will fall into the 1 mile deep ravine without the torch being on. The team of four cross at different speeds, we’ll call them A, B, C and D. A takes 1 minute, B 2 minutes, C 5 minutes and D 10 minutes. Bearing in mind only two may cross at a time, and the torch must be ON to cross, and is NOT allowed to be thrown back across the bridge, how do all four get across safely?

I’ll have a pint of your best please barkeeper

Gedge

*Pours for Gedge while distractedly considering the problem.*

*Brings a for Gedge.*

A and B cross = 2 minutes, total = 2 minutes
A returns = 1 minute, total = 3
C and D cross = 10 minutes, total = 13
B returns = 2 minutes, total = 15
A and B cross = 2 minutes, total = 17.

OK, how about this, I assume that as two people can cross the torch only has to be held by one of them on the bridge:

Give the torch to D who will spend 10 minutes on the bridge, this gives A, B and C time to cross one after the other before D reaches the other side.

Of course, I may be wrong, I was never very good at these type of things .

OK, I think I prefer Amy's solution. Very neat Amy

Thank you

*takes a bow*

The trick is to spot the counterintuitive move that results in A and B having to cross twice.

Very good Amy, took me an hour

Gedge

I like BS's solution, much less running about.

me too

Much more civilised, especially when in a hurry.

Couldn't they just use their mobile phones to call for a helicopter? If not then it's their own fault for not having planned their route more carefully and arrived at the bridge before it got dark so I have no sympathy for them...unless one has white chocolate in which case we should form a rescue party and help them immediately...oh wait this is hypothetical isn't it

You never know though, one might have had some white chocolate.

I've come to the conclusion that anyone who's sensible enough to buy white chocolate wouldn't be stupid enough to get caught on one side of the bridge...also they'd just steal the torch and run across the bridge to save themselves and the chocolate

I personally prefer dark chocolate, but any kind will do.

Perhaps the rest of the group would lend me their chocolate if I let them borrow my torch.

I have 1 500g bar of dark chocolate and 5 torches (all with spare batteries)

That's definitely the best solution so far.

Just don't give me the chocolate till we are on the other side of the bridge, otherwise I wont make it.

That's OK. You hold my torch and I will hold the chocolate.

*mutters to self*

Its dark chocolate, she wont eat it
Its dark chocolate, she wont eat it
Its dark chocolate, she wont eat it
Its dark chocolate, she wont eat it
Its dark chocolate, she wont eat it
Its dark chocolate, she wont eat it
Its dark chocolate, she wont eat it

Who said I didn't like dark chocolate. I love the stuff, the darker the better. And this is Lindt's Excellence, my favourite.