Here is a puzzle I came up with.
Ali and Billie are approaching a river from one side, and Casey and Devin from the other side.
Ali shouts: "Do you two also want to cross?"
Casey shouts back: "Yes! This should be no problem, there is plenty of boats on each side, and it takes just one minute to cross."
Devin shouts: "I don't know, the boats seem rather difficult to steer. I will only go if either Ali or Billie are in the boat."
Billie shouts: "They're also pretty small. I will only go with Devin alone."
Ali shouts: "I'm OK as long as I'm not alone with Casey in the boat!"
What is the fastest way for everyone to cross the river?
I'd also be interested in ideas on how to improve the story (the constraints seem a bit arbitrary...)