Puzzle: Four travellers are travelling in the mountains. It is night-time and they are nearly at their destination. The only thing in their way is a ramshackle old bridge, which is only able to support two people at a time. The travellers only have power left in the batteries of one torch, and they need the torch to be able to cross the bridge. The travellers are in different physical conditions after a long day of hiking. This means that they all take different amounts of time to cross the bridge. The times for each person are as follows: 2 min, 6 min, 9 min, and 14 min. They have a booking at an Airbnb that is a 10 minute walk from the other side of the bridge, which they can check into in exactly 51 minutes. How early will they be assuming that they know the fastest way to cross the bridge?
Cache: the cache is an opaque film canister. There is a bit of rocky terrain at the GZ. Please BYOP
You can validate your puzzle solution with certitude.