Quote:
Originally posted by lordjeebus
30 seconds: light both ends of one rope simultaneously -- because it is lit twice and the burns don't meet until the rope is gone, the rope disappears twice as fast. I don't think any additional information is necessary for this answer to work.
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Actually, you are assuming (as I have said, before) that the rope burns symmetrically.
Quote:
are the ropes identical?
EDIT: if they are identical, you can use the saltfish method, and when the burns line up (ie. after 30 sec), you can light the non-burning end of one of the remaining pieces. It will burn up 15 sec. later.
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Again,
saltfish's method only works if the rope burns at a constant rate.
If we may assume that the ropes burn symmetrically then we can get 15 seconds from burning the first rope from both ends while burning the second rope from a single end. When the first rope finishes burning, 30 seconds will have passed. After 30 seconds, if we stop the second rope from burning and then re-burn what's left from both ends then we will have our 15 seconds!
The ropes needn't even be identical!