__Puzzle:__

A man has two ropes of varying thickness (Those two ropes are not identical, they aren’t the same density nor the same length nor the same width). Each rope burns in 60 minutes. He actually wants to measure 45 mins. How can he measure 45 mins using only these two ropes.

He can’t cut the one rope in half because the ropes are non-homogeneous and he can’t be sure how long it will burn.

__Puzzle Solution:__

He will burn one of the rope at both the ends and the second rope at one end. After half an hour, the first one burns completely and at this point of time, he will burn the other end of the second rope so now it will take 15 mins more to completely burn. so total time is 30+15 i.e. 45mins.

well done

fold the first rope once.

fold the second rope twice.

now burn the first rope from one end after it completes burning it will be 30 mins

now start burning second rope.

as it is folded twice (60/2)/2=15 it will burn for 15 mins.

Why even to use the second rope? Take the first rope and fold it 4 times (ofcourse matching the ends with every fold). Burning 3/4 of this folded rope would give you 45 min.

FYI : This is exactly how I put this in Blackrock Interview. However the interviewer wasn’t very impressed. I think she already had made up her mind that the solution posted above is only the correct solution to this problem.

That is because the ropes are not of same length and density throughout, so it may be denser at the beginning and thinner at last, so if that is the case it takes much time to burn the beginning part than the last.

But then in that case burning the rope from both the ends would not give the exact time either.

yeah I Had the same question..!

Folding won’t help as ropes are not identical(non homogenous). You can’t be sure of how

The actual answer to this question would be something like this.

Take first rope and burn it at both the ends and burn one side of the rope 2.

By the time rope one burns it will be 30 mins. Rope 2 would also burn to a point where only 30 mins are left for it to burn completely.

At this point burn the rope 2 at the remaining end and it will burn off completely in 15 mins.

So the total time can be calculated like below.

First rope + Second 30 mins + Second rope 15 mins = 45 mins.