Play · Tricky
Two burning ropes
Two ropes and a lighter. Each rope takes exactly 60 minutes to burn end to end, but unevenly, so half a rope isn't half an hour. You have no clock. Ring the bell at exactly 45 minutes.
The puzzle
burned out
Rope Aburned out
Rope BLight whichever ends you like, then start.
How to think about it
Trust only the guarantee
Don't trust the rope's length; only its total burn time is guaranteed. A rope lit at both ends burns out in exactly 30 minutes, wherever the flames meet. If the other rope was burning from one end the whole time, it has exactly 30 minutes left at that moment. What happens if you light its other end too?