"Four men crossing a bridge" is a logic puzzle that I came across a few years ago. I particularly enjoy this puzzle because it appears simple at first glance but becomes deviously complex as you attempt to solve it. I did not make it up, and take no credit for it's invention. Unfortunately, I am unaware of it's origin.

Four men are travelling through a forest when they come upon a rickety old bridge spanning a gaping chasm. The bridge can only hold the weight of two men at once, and it is so dark that a flashlight is necessary to successfully navigate. Since the men only have one flashlight among them, they will keep sending two men across the bridge and have one bring the light back until all four are across. The four men are of different physical ability, and they can cross the bridge in one minute, two minutes, five minutes and ten minutes respectively. If two men cross the bridge, they must both travel the speed of the slower man. For example, if the one minute man and the ten minute man travel across together, it will take them both ten minutes to get across. What is the fastest time all four can successfully cross the bridge?

Although lateral thinking is a good thing, it is not used to solve this puzzle (i.e. the men do not use the flashlight to signal a helicopter to take all four across in one minute.) There are no word tricks here, just try to figure out the most efficient use of each man's ability to span the bridge. Feel free to add your own solutions to this node, and we'll see who can come up with the fastest time.