The second brain teaser is one of the river crossing puzzles. It’s another example in my opinion of an initially counter intuitive puzzle which becomes simple once you understand what assumption you should (or should not) be making. It’s most interesting in my opinion if you don’t know in advance in what time it can be solved, but instead try to figure out the minimal possible solution. I’ve seen a few people be utterly convinced their solution was the right one (unable to get out of the idea there might be a better one). Also note that with many of these problems it becomes much easier if you write down the options.
Four people come to a river in the night. There is a narrow bridge, but it can only hold two people at a time. They have one torch and, because it’s night, the torch has to be used when crossing the bridge. Person A can cross the bridge in one minute, B in two minutes, C in five minutes, and D in eight minutes. When two people cross the bridge together, they must move at the slower person’s pace. The question is, what is the fastest they can all get across?
