Brainteasers

[Oyvoyvoyv]

TruckOSaurus said:
I've got a new one :

You've got four people that need to cross a bridge : A sprinter, a regular guy, a fat guy and an old grandma. However the bridge is not in good shape so only two people can cross it at once. It is also pitch black and you need a torch to cross the bridge. Each pair can only go as fast as the slowest of the two.

The sprinter can cross the bridge in 1 minute
The regular guy can cross the bridge in 2 minutes
The fat guy can cross the bridge in 5 minutes
The old grandma can cross the bridge in 10 minutes

What's the quickest way to get everyone across?

Okay, just to start off. (I leave all my ideas so others can watch and get new ideas on my failed attempts) 

If you simply did

> Sprinter + regular = 2 minutes

< Sprinter alone = 1 minute

> Sprinter + Fat Guy = 5 minutes

< Sprinter alone = 1 minute

> Sprinter + grandma = 10 minutes

 

So that's 19 minutes. We have to figure out a way to beat that.

So, here's an idea.

 

> Sprinter + Regular = 2 minutes

< Sprinter = 1 minute

> Fat guy + Grandma = 10 minutes

< Regular = 2 minutes

> Sprinter + Regular = 2 minutes

That's 17 minutes.  

 

> Sprinter + Regular = 2 minutes

< Regular = 2 minutes

> Fat + Grandma = 10 minutes

< Sprinter = 1 minute

> Regular + Sprinter = 2 minutes

That's really the same thing, and also gives 17 minutes.