Measure Time by Burning Rope Puzzle-Asked in interviews.


You have two ropes and a box of matches. Each rope takes exactly one hour to burn, but they may not necessarily burn evenly — i.e., the first half might burn in the first 10 minutes and the second in the remaining 50). What durations of time can you measure using the 2 ropes? Times do not have to start when the first rope is lit.

A) 1 hour

B) 30 minutes

C) 45 minutes

D) 15 minutes

Solution: A B C D are the solutions

Burining a single rope lets you measure 1 hour. Burning 2 ends of it at the same time let you measure 30 minutes.
Start by lighting both ends of one rope and only one end of the second rope. When the first rope is finished burning (i.e., after a half an hour), light the other end of the second rope. At this point, the second rope is halfway burned, so it will take another 15 minutes to burn the second half with both ends lit. When the second rope is done burning, 45 minutes has elapsed.
15 minutes has elapsed after first rope is completely burnt.

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