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I only have two candles, each candle burns for exactly one hour. I do not know the length of the candle. Guessing and estimation is not allowed. You are not allowed any other materials other than the two candles (and a lighter I suppose).

EDIT:

I made a mistake, the candles burn for two hours, not one.

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  • $\begingroup$ not an exact duplicate, the ratios are different. $\endgroup$
    – Jasen
    May 8, 2016 at 12:16
  • $\begingroup$ Agreed @Jasen ${}$ $\endgroup$
    – warspyking
    May 8, 2016 at 12:32
  • $\begingroup$ @Jasen The ratio is different, but the solution works the same way. The only difference is that you start counting time when the first candle/rope is finished. $\endgroup$
    – f''
    May 9, 2016 at 7:06
  • $\begingroup$ @f'' doesn't work after the 2 hour edit. $\endgroup$
    – Jasen
    May 9, 2016 at 7:09
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    $\begingroup$ @Jasen It works perfectly fine after the 2 hour edit. $\endgroup$
    – f''
    May 9, 2016 at 7:11

2 Answers 2

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Light one candle at both ends. When it is done burning, half an hour has passed.
Do what you want with the other candle. You don't need it.

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  • $\begingroup$ well, ideal candles may burn equally fast at both ends, but real candles seem to burn at an accelrated rate due to wax dripping off. $\endgroup$
    – Jasen
    May 8, 2016 at 4:55
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    $\begingroup$ @Jasen: It doesn't matter whether it burns at the same speed. As long as the whole candle takes an hour, then the whole candle from both sides will take half an hour. $\endgroup$
    – Deusovi ♦
    May 8, 2016 at 5:11
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If you are in a hurry.

balance one candle across the other (lying down), where it is balanced is half its length cut or mark it there, then burn it to the mark.

if you have plenty of time to prepare

do the above to get two "half" candles , bur both of them and the whole candle, if one half burs out, divide the remainining half-candle, When the split candle is entirely consumed the whole candle will be half burned, extinguish it and save it for when you need to time half an hour.

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  • $\begingroup$ Sorry, Jasen. This approach requires that you estimate half and I explicitly said that the answer had to be exact and could not use estimation. Good try though. $\endgroup$
    – Oreo
    May 8, 2016 at 4:43
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    $\begingroup$ Thats first option is a measurement, not estimating. perhaps you should update your problem with a list of what actions are allowed. $\endgroup$
    – Jasen
    May 8, 2016 at 7:51

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