June 17, 2010


You have two ropes, each of which takes two hours to burn if lit at one end. These ropes are not homogeneous and some parts of the rope burn more quickly than the other parts. If you cut the rope into half, you cannot assume that the half rope will take one hour to burn.

Use these ropes to measure 1hr 30 min.


Set fire to both ends of one rope. At the same time, set fire to one end of the other rope. The rope lit from both ends will take one hour to burn completely (it is burning at twice the rate). Just as it finishes burning, the other rope would have burned for one hour with one hour remaining. Now light this rope from the other end as well, so that the remaining part burns in 30 minutes.


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