Measure 45 minutes with two ropes
You have two ropes. Each takes exactly 60 minutes to burn, but neither burns at a uniform rate. With a lighter and no clock, how can you measure exactly 45 minutes?
Lateral challenge at medium difficulty.
Hint
Burning a rope from both ends changes its total burn time even when its rate is uneven.
Answer and method
Light both ends of the first rope and one end of the second. The first rope finishes after 30 minutes. At that moment, light the other end of the second rope. Its remaining portion now burns from both ends and finishes 15 minutes later. Total: 45 minutes.
Reusable reasoning moveTransform an unreliable rate into a reliable completion event.