Questions In Interview

Interview Question: The Burning Rope

There are two ropes, each rope takes 1 hour to burn. But either rope has different densities at different points, so there’s no guarantee of consistency in the time it takes different sections within the rope to burn.

How do you use these two ropes to measure 45 minutes?



Strategy and Approach to Answering a Problem Solving Interview Question

If you get asked a problem solving question in an interview, remember that they are NOT looking for the RIGHT answer. They are evaluating the approach you use to think out and solve the problem. A strong candidate demonstrates the following:

  1. Evaluates and understand the scope of the problem
  2. Communicates assumptions
  3. Demonstrates quantitative analytical skills
  4. Answers the question that has been asked. You’d be surprised how many candidates get lost in the analysis and solve for a different question than what was asked.


How to answer this problem solving interview used by Google or Microsoft

We know that it takes 1 hour to burn each rope.  Each rope has 2 ends.  If we light the rope simultaneously on both ends, it will take half the time to burn the rope (30 minutes).

Therefore the solution is to:

  1. Burn 1 rope simultaneously on both ends and light the other rope on one end only (30 minutes)
  2. After 30 minutes, one rope is burnt and the other rope has 30 minutes of burn time left.  Therefore, if we now lit the other end of that rope so the total burn time left would be 1/2 or 15 minutes.  (15 minutes or 45 minutes total time)

Note: For this problem solving job interview question, there is an actual logical answer, however the interviewer will want to evaluate your thinking process. In answering the interview question, the lead up to your answer will be just as important as the actual answer itself.

For more problem solving, brain teasers, riddles, puzzles, logic job interview questions, please check our out Problem Solving Interview Section.


List of Problem Solving Interview Questions Used by Top Firms (such as Google or Microsoft)