Top Puzzle 10: The Two Ropes Burning Puzzle | Puzzles for Interview - Interview Preparation PDF Download

Introduction

Are you ready for another brain teaser? This week's riddle is all about burning ropes and measuring time. The problem may be short, but finding the solution can be quite challenging. Let's take a closer look.

Problem

Imagine you have two ropes coated in oil to help them burn. Both ropes will take exactly one hour to burn all the way through, but they burn at inconsistent rates. This means that some parts of the ropes burn faster than others, while some parts burn slower. Your task is to measure exactly 45 minutes using only a lighter to ignite the ropes. How can you do it?

Solution

At first, you might think that you can measure 75 percent of the way down one rope and call that 45 minutes. However, because the ropes burn at inconsistent rates, this method won't work. There is a better way.
Here's what you need to do: Light one of the ropes on fire at both ends at the same time. This way, the rope will burn up in just 30 minutes, even if one side burns faster than the other. Meanwhile, light the second rope on one end only.
When the first rope burns out after 30 minutes, immediately light the unlit end of the second rope. Since 30 minutes of the second rope have already been used up, only 30 more minutes remain. At this point, remember that this doesn't necessarily mean that half of the rope's length has been burned. It could be more or less.
Light the other end of the second rope when the first rope burns out. This will cause the remaining part of the second rope to burn up in just 15 minutes. Once the second rope has been consumed by the flames, exactly 45 minutes will have passed.

Conclusion

The Burning Rope Problem may have seemed tricky at first, but hopefully, this solution made sense to you. This riddle teaches us to think creatively and find alternative solutions to problems.