The problem is this: Four people (let's call them alice, bob, charlie and david) need to travel through a narrow tunnel. Unfortunately, they only have one lightsource amongst them, and they're all too scared to move about in the dark.

In addition, the tunnel will only admit two people at once (it's very narrow[*]).

Now, the people travel at different rates. A group of two people must travel at the rate of the slowest. Given that it takes them 1, 2, 7 and 9 minutes to traverse the length of the tunnel (respectively for a, b, c, d) the question is this: how quickly can they all get from one end to the other?

(By which, munchkins, I mean they must all _end_up_ at the far end of the tunnel.)

[*] You want a better couching of this? Ok, it's very narrow and subject to structural flaws which may cause it to collapse if more than two people are in it at once. So there.