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[Brain Teaser] Ropes & Measuring Time

Question:

  1. You are given a rope which can be fully burnt in one hour. The rope burns unevenly meaning half burnt does not necessarily indicate a half-hour mark. Using this rope, how would you know that 30 minutes has passed?
  2. If you were given two ropes that burn unevenly, how would you measure 45 minutes?

 

Interview Questions

 

Answer:

  1. To measure 30 minutes, burn both ends at once and even though one side will burn faster than the other, the point at which they meet is when 30 minutes have passed.
  2. Similar to the answer above, you now know how to measure 30 minutes. For two ropes, take one rope and burn both ends and take the other rope and only burn one end. When the first rope burns out, that is the half-hour mark – when the first rope burns out, immediately light the opposite end of the second rope. When the second rope burns, you know that 15 minutes have passed since the half-hour mark.
Image Source: relativelyinteresting.com
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