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Getting it down on `paper`

Burning Rope for 45 Minutes

Premise: There are two ropes of equal length and physical composition. Each rope takes one hour to burn from end-to-end, but may burn at a variable rate. You are given a lighter and these two ropes. There is nothing else that can be utilized, not sand nor clock nor sun.

Question: How can you determine when 45 minutes have elapsed?

Answer: Light one rope at both ends. Light the other rope at only one end. When the rope that was burning from both ends has burnt out, 30 minutes have passed. Light the second end of the rope that was only burning from one end. This rope will burn out in 15 more minutes. The total time will have been 45 minutes.

 

Comments: 1

 
 
 

“When the rope that was burning from both ends has burnt out, 30 minutes have passed.”

That doesn’t follow from the stated properties of the rope. Without further information, this riddle has no solution.

 
 

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