Logical Interview Question

[RonBW]

What I find astounding is that we've spent 1000 years developing a sophisticated education system to test people's IQs with entails two decades of education with objective measures at the end of it, and these interviewers reduce it to an "anal" question.

Those can be the most painful ones to answer though. Just ask my gf.

[LondonManc]

I figured you could light the first rope at both ends. Should average out at 30 minutes. Then, cut the other rope in half and then light at both ends which should give you the additional 15 minutes.

Though, to me as a sometimes overly literal person, I absolutely hate this questions. The very fact it says "burns inconsistently", implies you can't use any outside tools and then says "Measure exactly" makes it a bulltulip question.

Take his calibrated spirit level and make that shelf perfectly flat with no other tools. Stupid.

Cutting the other rope infers that you have outside tools.

Lighting three of the four ends and then lighting the fourth end when the first rope burns completely is a perfectly logical way of completing the task and accurate so long as your rope lighting skills are adequate (exact is relative - exact second is sufficient for this but rubbish for Formula 1 for example; similarly a 95% success rate is acceptable in driving tests but not in airplane landings).

[OwlHoot]

==
You have two ropes.

Each takes exactly 60 minutes to burn.

They are made of different material so even though they take the same amount of time to burn, they burn at separate rates.

In addition, each rope burns inconsistently.

How do you measure out exactly 45 minutes?
==

The word "inconsistently" implies that to make a reliable measurement you must burn one or both ropes entirely, and thus measure only full 60 minute intervals.

I don't see how any scheme such as lighting the rope at both ends or cutting it can work because in theory all but a small lump of rope could burn almost instantly like a fast fuse leaving the last bit to smoulder away for essentially the full hour.

Unless the original question said "consistently" ?!

[vetran]

The word "inconsistently" implies that to make a reliable measurement you must burn one or both ropes entirely, and thus measure only full 60 minute intervals.

I don't see how any scheme such as lighting the rope at both ends or cutting it can work because in theory all but a small lump of rope could burn almost instantly like a fast fuse leaving the last bit to smoulder away for essentially the full hour.

Unless the original question said "consistently" ?!

Indeed.

The question & answer are contrived.

[LondonManc]

The word "inconsistently" implies that to make a reliable measurement you must burn one or both ropes entirely, and thus measure only full 60 minute intervals.

I don't see how any scheme such as lighting the rope at both ends or cutting it can work because in theory all but a small lump of rope could burn almost instantly like a fast fuse leaving the last bit to smoulder away for essentially the full hour.

Unless the original question said "consistently" ?!

If you light the rope from both ends it burns in half an hour, no matter how inconsistently it burns along its entire length.

You've lit one end of the other rope at the same time as the two ends of the original. By lighting the other end of the rope upon the complete burning of the first rope, you know that it will burn in 15 minutes, no matter what length is left because it is half way through burning. Once that is burned through, you have reached 45 minutes.

[OwlHoot]

If you light the rope from both ends it burns in half an hour ...

That would be true if it burned uniformly, or "consistently".

But as I said suppose the rope comprises almost entirely a highly inflammable "fluffy" substance that all burns in a flash to nothing in say a couple of seconds, and a small blob of dense waxy stuff that smoulders for an entire hour.

Whether or not both ends of the rope are lit at once, or the rope is folded in half or doubled up again, or however it is coiled up, what will happen is the fluffy stuff will go up and vanish almost immediately, leaving the waxy blob to smoulder for the full hour.

Also, one can't assume that each time the rope is lit the resulting combustion somehow acts independently of any previous combustion already started, even where they overlap. Otherwise you could light a normal candle a hundred times in quick succession and practically turn it into a stick of dynamite!

In other words overlapping combustion is idempotent, in that subsequent lightings or converging combustion areas have no further effect on it.

[WTFH]

That would be true if it burned uniformly, or "consistently".

But as I said suppose the rope comprises almost entirely a highly inflammable "fluffy" substance that all burns in a flash to nothing in say a couple of seconds, and a small blob of dense waxy stuff that smoulders for an entire hour.

Whether or not both ends of the rope are lit at once, or the rope is folded in half or doubled up again, or however it is coiled up, what will happen is the fluffy stuff will go up and vanish almost immediately, leaving the waxy blob to smoulder for the full hour.

Also, one can't assume that each time the rope is lit the resulting combustion somehow acts independently of any previous combustion already started, even where they overlap. Otherwise you could light a normal candle a hundred times in quick succession and practically turn it into a stick of dynamite!

In other words overlapping combustion is idempotent, in that subsequent lightings or converging combustion areas have no further effect on it.

So if your waxy blob burns for 50 minutes when burning from one end only, how long will it last if it burns twice as fast?

It doesn't matter about inconsistency. Lighting 1 end will mean it will burn for 1 hour in total. Lighting both ends means the total burn time will be half that.
The two burning ends might not meet in the middle, due to inconsistencies, but the burn time will be 30 mins.

...and so we come to the candle. You can either burn a candle from one end, or you can burn it from both ends. If you burn the candle from both ends, it burns out twice as fast as if it was only lit from one end.
Let's imagine that lighting one end of a candle wick produces one candela of light.
Light both ends of the wick, you get two candelas.
Have a candle with multiple wicks... you get more light when they are all lit.
And to stop multi-wick candles from burning too fast, they put loads of wax on them.

[LondonManc]

That would be true if it burned uniformly, or "consistently".

But as I said suppose the rope comprises almost entirely a highly inflammable "fluffy" substance that all burns in a flash to nothing in say a couple of seconds, and a small blob of dense waxy stuff that smoulders for an entire hour.

Whether or not both ends of the rope are lit at once, or the rope is folded in half or doubled up again, or however it is coiled up, what will happen is the fluffy stuff will go up and vanish almost immediately, leaving the waxy blob to smoulder for the full hour.

Also, one can't assume that each time the rope is lit the resulting combustion somehow acts independently of any previous combustion already started, even where they overlap. Otherwise you could light a normal candle a hundred times in quick succession and practically turn it into a stick of dynamite!

In other words overlapping combustion is idempotent, in that subsequent lightings or converging combustion areas have no further effect on it.

You're over thinking it. It's a flammable rope that burns inconsistently. As soon as you talk about folding it in half, you're lost; the concept of half an hour only exists on the time front, not the length front in this case. The two lots of burning might meet in exactly the middle or, more likely, not. It doesn't matter. The point is that they will meet after half an hour; burn it from one end and it takes an hour. Burn it from the other and takes an hour too. Burn it from both ends and by definition, they'll meet after half an hour.

[northernladyuk]

They gave me the following question which I solved my way and not the "right" way they suggest on here https://www.brainbashers.com/showpuz...sp?puzzle=ZVGA

==
You have two ropes.

Each takes exactly 60 minutes to burn.

They are made of different material so even though they take the same amount of time to burn, they burn at separate rates.

In addition, each rope burns inconsistently.

How do you measure out exactly 45 minutes?
===

My solution was to use the first rope to map its hour-burning interval to a repeating event which event I would repeat over and over for the duration of the burning. For example over 1 hour I may get 200 repetitions of the event and then figure out how many repetitions equal to 45 mins and this way solve the puzzle.

I still stand by my answer because the officially suggested one (see the link ) seems wrong because of the fact the rope doesn't burn at equal speed...

Cutting the other rope infers that you have outside tools.

Lighting three of the four ends and then lighting the fourth end when the first rope burns completely is a perfectly logical way of completing the task and accurate so long as your rope lighting skills are adequate (exact is relative - exact second is sufficient for this but rubbish for Formula 1 for example; similarly a 95% success rate is acceptable in driving tests but not in airplane landings).

How do you propose to set fire to the rope?

[radish2008]

Interviewer: Describe your biggest weakness.

Candidate: Honesty

Interviewer: IO don't think honesty is a weakness ...

Candidate: I don't give a tulip what you think.