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Burning Ropes
JP Morgan
2 different ropes burn at different speeds in different sections, but they both take exactly 2h to burn. How do you measure 30 min of time?
Answer
We need to somehow halve the 2h time twice to get to 30 min. We can halve a time of the rope's burning by lighting it from both sides. So light one rope up from both sides and the other from one side only. The one lit up from two sides burns in 1h, and after when it does, the other rope still has 1h to go. Now we have a 1h rope effectively, so repeat the process: light this remaining portion of the rope from both ends - it will burn in exactly 30 min.