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?
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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