Measure 45 Minutes
Logic puzzles require you to think. You will have to be logical in your reasoning.
You have two ropes. Each takes exactly 60 minutes to burn. They are made of different material so even though they take the same amount of time to burn, they burn at separate rates. In addition, each rope burns inconsistently. How do you measure out exactly 45 minutes?
Answer
Take one rope and burn it at both ends.
At the same time, burn one end of the
other rope. When the first rope finishes
burning (ie. 30 minutes), light the other
end of the remaining rope (half of the
remaining 30 minutes gives you 15 minutes)
When it burns out, that`s 45 minutes.
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Comments
jimbo   
Mar 02, 2003
| Good puzzle. I got as far as the first part, burning both ends to get 30 min. I should have twigged how to get half of the half. Sometimes you don't see the obvious to light the second rope AT THE SAME TIME! |
boodler 
Nov 26, 2005
| Easy when you know how!  |
blonde_genius 
Jul 14, 2006
| ??? But how can you be sure? Since It doesn't burn at a consistant rate??? 
otherwise a good teaser, |
brudork
Mar 12, 2007
| That got me |
winbob423
Apr 03, 2007
| ooops........missed this one  |
Deedee123
Dec 31, 2007
| "You shouldn't play with fire!"  |
andreja
Feb 19, 2008
| I don't like this one. It doesn't make any sense (to me)
if a rope burns at random speed, it could burn very, very, very slowly for 59 min, and then very fast for 1 min.
but this teaser had great potential  |
JasonD
Aug 04, 2008
| andreja, It is implied that each rope takes sixty minutes to burn *if* it is burning from one end only.
But in the solution provided, the ropes are burned (at different times) from both ends.
So let's say the first rope has a "very, very, very slow" end which takes 59 minutes to burn a quarter of an inch. The remaining 13 inches (or whatever) will burn in 1 minute. Lighting both ends will therefore consume the "fast" section in a single minute, leaving 58 minutes of burn time for the remaing ~1/4 inch. Burning at both ends, this little stub will be gone in another 29 minutes.
Therefore, 30 minutes have passed, and the second rope has been burning the whole time, so it has 30 minutes of burn time left. Lighting the second end of that rope makes it burn twice as fast (another 15 minutes).
That help? |
neoteny
Mar 06, 2010
| blonde_genius and others are right: if the ropes burn inconsistently we cannot necessarily measure 45 minutes. JasonD isn't taking the word "inconsistently" seriously enough.  |
cluemaster
Sep 19, 2011
| Inconsistent burning rate is irrelevant, so long as the total burn time is still 60 minutes. If you were measuring out the rope and burning half, then it would matter, but it doesn't in this puzzle.
It's like programming with concurrent threads. Or think of it like projects and man-hours.
Let's say you have 2 projects and they each take 60 man-minutes. If you put two people on the first job, it will be done in 30-minutes. If at the same time, you'd put a third person on the second project, that project would be half done when the first project finished, so it would only take 30 more man-minutes to complete. So if you add another person, that's only 15 minutes and it's done after a total of 45 minutes. |
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