Camp Counselor
Source: a friend
You are a camp counselor. On a hiking trip, you and your 8 kids are making it back to the campsite when you get somewhat lost. There are four trails to choose from, and only one of them leads back to the campsite. The campsite is 20 minutes away, so if you walk down a trail for 20 minutes and don't see the campsite you can assume it's the wrong trail. There are only 60 minutes until it gets dark. It'll take 20 minutes to walk down a trail to check it, 20 minutes to walk back, and then there are only 20 minutes left to get back to camp. Since you yourself can only check one trail, you'll need to enlist the help of your happy campers.
Trouble is, two of the campers (you don't know which two) are unreliable, and may sometimes (though not always) lie about what they have seen. How can you ensure that everyone gets back to camp before dark?