hardlogic

Illustration for riddle: You have two ropes. Each rope can burn in exactly one hour. The ropes are not of the same length or — Can you figure out this riddle?

You have two ropes. Each rope can burn in exactly one hour. The ropes are not of the same length or width. Both ropes are also not of uniform width or thickness. The ropes are thick at certain places and thin at other places. Thus, a rope that is half burnt will not necessarily have taken 30 minutes to burn. By burning the ropes, how do you measure exactly 15 minutes worth of time?

Let's name the ropes as A and B. Light A from both the ends and B from one end. When A is finished burning we know that 30 minutes have elapsed and B has 30 minutes remaining. Now, light the other non-burning end of B and start the timer. When B is finished burning, stop the timer as 15 minutes have elapsed.

Why this works

This riddle hinges on the unpredictable burn rate of the ropes while still leveraging their total burn time of one hour. By lighting Rope A from both ends, you ensure it burns completely in 30 minutes, regardless of its irregular thickness. Meanwhile, Rope B, initially lit from one end, will have 30 minutes of burn time left when Rope A finishes. Lighting the other end of Rope B at that moment allows it to burn out in half the remaining time, which is exactly 15 minutes. Thus, through clever timing and manipulation of the burn rates, you can accurately measure a quarter of an hour!

Common Wrong Answers

“Burn both ropes from one end at the same time.”

Burning both ropes from one end would mean that they would each take one hour to burn completely, making it impossible to measure a shorter time interval like 15 minutes.

“Burn rope A from both ends and rope B from both ends.”

If you burn rope B from both ends as well, it will also take 30 minutes to burn completely, but you won't be able to measure the 15 minutes accurately once both ropes are completely burnt.

“Cut the ropes in half and then burn from one end.”

Cutting the ropes in half does not guarantee that they will burn in a predictable time due to their non-uniform thickness. This method fails to provide an accurate measurement of 15 minutes.

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