You have two ropes that each take 60 minutes to burn completely, though they burn at non-uniform rates. How can you use these ropes and a lighter to measure a precise interval of 45 minutes?
Light both ends of the first rope and one end of the second rope simultaneously. When the first rope burns out completely, light the other end of the second rope.
Lighting both ends of Rope 1 makes it burn in 30 minutes. At that point, Rope 2 has 30 minutes of burn time left. Lighting its second end makes it burn out twice as fast (15 minutes), totaling 30 + 15 = 45 minutes.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Hard. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
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