Mental Floss

Brain Game: Bridge for Four


There are few things less fun than having your car conk out in the middle of a rural area when it's dark and cloudy (even if you do have AAA roadside assistance). Here's a puzzle about four women who get stuck in that situation:

Four women are in a car that runs out of gas very late one night. For safety's sake, they decide to abandon the car as a group, since they have only one flashlight between them. They see a light on at a nearby farmhouse, but none of the women want to approach it alone. In order to reach the house, they must make their way across a creaky old footbridge that will support the weight of no more than two of them at a time. The women must carry the flashlight while crossing the bridge, because it's dark and very difficult to see. Andrea is the slowest; it takes her ten minutes to traverse the bridge. Barbara can do it in five minutes, Carolyn can make it in two minutes, and speedy Delilah can cross in only one minute. Of course, if two of the women cross simultaneously, they do so at the slower woman's pace. Can you figure out a way to get all four women across the bridge in only 17 minutes?

HERE is the solution.


First, Carolyn and Deliah cross the bridge (which takes 2 minutes).
Then, Carolyn crosses back (another 2 minutes).
Andrea and Barbara then cross together (which takes 10 minutes).
Delilah crosses back (which takes 1 minute).
Finally, Carolyn and Delilah cross the bridge once more (which takes 2 minutes), for 17 total minutes.