The Burning Candles Puzzle

Interview puzzles are critical-thinking and logical reasoning problems that make candidates use their problem-solving and analytical skills to find a solution. Interviewers prefer to ask these puzzles because it helps them understand your thought process, and your ability to solve complex problems using strategies, and high-order thinking. Usually, candidates applying for software development, data science, or engineering roles may encounter such puzzles. Much like brain teasers and logical reasoning questions, puzzles for interviews may not directly relate to your job role.

Here is an Interview puzzle that is frequently asked in Job Interviews.

Problem Statement:

There are two candles of equal lengths but of different thicknesses.

The thicker candle lasts for 6 hours while the thinner one lasts for 2 hours lesser than the thicker one.

A person lights the two candles at the same time and goes to play.

After returning home, the person saw that the thicker candle was twice the length of the thinner one. How long ago did the person light the two candles?

Solution:

According to the problem statement, the thicker candle lasts for 6 hours while the thinner one lasts for 2 hours lesser than the thicker one.

Therefore, the thinner candle lasts for 4 hours.

The problem reduces to finding the number of hours it takes for the thicker candle to become twice the length of the thinner candle.

Analyzing the problem hour by hour, after each hour (1/6) part of the thicker candle is melted while (1/4) part of the thinner candle is melted.

After 1 hour, (5/6) part of the thicker candle remains while (3/4) part of the thinner candle remains.

After 2 hours, (4/6) part of the thicker candle remains while (2/4) part of the thinner candle remains.

After 3 hours, (3/6) part of the thicker candle remains while (1/4) part of the thinner candle remains.

On observing carefully, it can be found that at the end of 3 hours the remaining part of the thicker candle is (1/2) while it is (1/4) for the thinner candle i.e., the thicker candle is twice the length of the thinner one.