You have two ropes. Each takes exactly 60 minutes to burn, but they burn unevenly. How can you measure exactly 45 minutes?
Hint: Light a rope from both ends.
Light rope A at both ends and rope B at one end. When A finishes (30 min), light B's other end. When B finishes, 45 minutes have passed.
A rope lit at both ends burns out in half the time no matter how uneven it is. B has 30 minutes left at the halfway point, and lighting its other end halves that to 15.
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