[engineer question] burning rope

[martyr]

this is a recruiting tool my dad uses when hiring support/software engineers. if you're looking for a job in bangalore, reading this thread is cheating.




you have a length of rope and an ignition source. the rope is known to burn for exactly 60 minutes end to end.

with no other tools, indicate the passage of 45 minutes

[Rahl]

And thats where quality software comes from.

Obviously the answer is that you should go to a third world nation, find some desperate people, use the rope to tie them to a chair, threaten to burn them with the ignition source unless they develop perfect timekeeping skills so they can tell you when 45 minutes has passed.

[Navin]

Problem definition is incomplete - there are an infinite set of solutions as stated.

Missing data - burnrate = f(t); state the function, which determines the answer.

A priori concept - Hire an American...use third world countries as test sites for global domination weapons.

[SirKibbleX]

Heh, that's interesting, because I had an idea to genetically engineer the zerg from Starcraft and introduce them to Uzbekistan.

[martyr]

Problem definition is incomplete - there are an infinite set of solutions as stated.

Missing data - burnrate = f(t); state the function, which determines the answer.

f is undefined and inconsequential, as f(t) is a constant. (linear burn rate)

[elana]

this is a recruiting tool my dad uses when hiring support/software engineers. if you're looking for a job in bangalore, reading this thread is cheating.




you have a length of rope and an ignition source. the rope is known to burn for exactly 60 minutes end to end.

with no other tools, indicate the passage of 45 minutes


Take the rope. Fold it in half, then fold it in half again. Now, you should have 4 lengths of rope, each being 15 minutes in burn length. Mark where one of the last lengths is. So it looks like this:

----+----+----+----
^


Light at the mark like I indicate above. It will burn to the left end in 15 minutes, but it will burn to the right end in 45 minutes.

So..... I hear bangalore is... in india this time of year...

Though I like Rahl's answer better :)[/code]

[Rahl]

Hah, see the difference between our answers Elana is that yours is the one they're looking for in developers, but my answer has upper-level management written all over it. If I'd given that answer in a real interview with martyr's dad I'd have a VP position paying 300K in no time.

[Rahl]

Oh, and is your dad's name Joel? Heh...

So, why are manwhole covers round?

[martyr]

same name as me. i'm the fourth, in fact.

the answer he wanted was to ignite the rope here:

----+----+----+----
.....^

(although i realize that it might not be rendering elana's spaces.)


and there's a part two to this question that makes it more interesting - but let's address rahl's hole first.

i would say, off the top of my head, that manhole covers are round because manholes are round.

[Numenor]

Manhole covers are round simply, so that they never fall through the hole. The circumference of the manhole cover is greater than the circumference of the hole and therefore, no matter which way you turn the cover it will still never fall through the hole. If it were square or any other shape and you turned it so that the flat edges of the cover were at the corners of the hole the cover would fall through. I can think of some ways to stop this from happening with some other shapes but using a circle is the easyest solution. Hope I was clear in my explanation.

[RogueVindicare]

Because squares are teh suck?

[Numenor]

wouldn't both elena's and your answers be right to the rope question? It is more a matter of looking at it from a glass half full or half empty. Using quarters of the rope to segment the measurement of the time is what your dad was looking for, was it not? Although, I do see how yours is more correct, only b/c you have no way of marking the rope and therefore, would have to hold it. When the flame started to get close to you hand you would have to let go b/c it was hot and then you would have to eyeball it.

[ArmorDeth]

Huh?

[martyr]

his would work, but burning the rope so that it's entirely gone is a better answer, representing the use of all avaliable resources.

now for part two:

you have two ropes.

again, their total burn time is, end to end, 60 minutes.

you need to have the last bit burn out at exactly 1:15 after initial ignition.

these ropes do not burn linearly.

[Navin]

My answer was most correct and wiser readers would have inferred I was providing an insight to problem solving. The reader has to make an assumption of linear burn rate to derive the simplistic answer of rope folding.

Premises and assumptions are to clearly described prior to stating conclusions - basic engineering training - otherwise the validity of the conclusion is limited.

I would be more impressed (if I was evaluating potential candidates) by a person who strived to understand the problem well-enough, establish the premises, state the assumptions then forumulate a conclusion with stated boundaries of validity.

To do otherwise is be sloppy, incomplete and in extreme cases misleading.

Later.

[Foudre]

Question 1: Why burn the whole rope and waste the leftover 15 sec timepeice if you dont have to? Take your ignition source and burn a mark at the 3/4 point.

Question 2: Impossible, I have no ignition source, you only gave me 2 ropes.

[Foudre]

Here is the question I was given during my interview (EE): There is a black box with 2 wires comming out of it providing high power. One does so in the form of high current while the other does so in the form of high voltage. Without using any form of measurement equipment how can you determine which is which?

Note: In the world of electronics "black box" just means some circuit we dont care about, all we care about are the outputs.

Note2 for non electrical people:
Voltage = Current x Resistance
Power = Current x Voltage

[Rahl]

Hah, to really understand these questions, you have to know who the employer is....

I'm gonna go with Navin though. I like his answer.

When I'm hiring engineers, I don't want guys who jump at the first clever little trick and consume all available resources. I want a guy who understands how to solve the general problem efficiently.

In general, if I were asked the question my answer: GET A FUCKING WATCH. Using a rope to measure time is stupid. If you only work with the tools placed in front of you you're a wuss. Go make, beg borrow or steal a better tool...the right tool. This is one of those "only have a hammer so everything is a nail" kind of situations. If you become comfortable using only the tools you're given or that are convientent for you, you're limiting yourself and are producing sub-par solutions. Now I'm angry...heh, I hate shitty engineering. I see too much of it.

[Rahl]

Oh, and manhole covers are round because men's holes are round.

[ArmorDeth]

Huh?

[martyr]

i won rahl's manhole.

for foudre's question: you ground the intern's left hand and have him/her grab one wire with the right hand.

if the intern dies, you've found the high-amperage wire.
if the intern's muscles all freeze, you've found the high-voltage wire.

for my questions: the idea behind asking the questions is to initiate some dialogue with the interviewer while the applicant puzzles.

part two of the rope question is also supposed to plant a seed of doubt - the interview, at this point, proposes that there may not be a solution.

[Rahl]

Foudre, the big wire has the high amp.

Or, you could use the stupid ropes. Tie the rope to a ground source and touch it to each wire...the one that causes the rope to catch fire has high amp.

There is always a solution. It just might not be attainable given the current tools....

[LuckYx]

if E=MC^2 then it's 1:56pm AK time and you are all a bunch of geeks...


haha just kidding

(now i know why i didn't goto PSUMA)

[snipegrzywa]

/\
|| LOL
||

[BuzzEvil]

Martyr:
You use the left over bit of rope from the first burn with the other two ropes twisted together to get an good average burn rate and that should equal the proper time.

Foudre:
The high voltage wire will arc to ground if you hold it close enough and if the voltage is high enough.
The high current wire cannot kill you if it has low voltage because amps = voltage / resistence ... the human body is very high in resistance and requires a high voltage to induce a lethal current flow. Thats why you can touch a car battery and not die and why they use high voltage with a high current combo to fry death row inmates.

If i didnt care about damaging the power source i would just ground one of the wires. The one that causes arc weld-like blasts on contact and possibly liquifies the conductor is the high current wire.

[elana]

Take one of the wires. Touch it to the guy interviewing. If he screams in agony, try the other wire. Repeat until he gives you the job.

[martyr]

haha, awesome

actually, there is a solution to the non-linear ropes, and it doesn't require any additional rope or torture.

[Navin]

While we are on this subject I think the most important people are engineers. The rest of humanity are parasites on backs of the technology developers and implementers.

It pisses me off to no end that multitudes of people end up in life with much, much more than the average engineer due to the basic idiocy of the masses. The masses reward selected, useless people(such as sports, video, music etc., "stars") with their resources creating icons of trivial, which in turn infect more of the masses.

The constitution should be amended to require a 2/3 majority of "technologists" be the membership - and only an engineer should be allowable as president.

[Rahl]

Navin for world dictator! Woo, yay for the power of the techno-witchdoctors!

[Numenor]

don't know if anyone figured out the second part of the question but here is my try....

you have the two ropes

tie the ropes together at the 3/4 point one and the 1/2 point on the other:
Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*|

Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*|
---+---+---+---
^Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*|

Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*|




when the rope gets to the 3/4 point in the first rope the second rope will catch fire and as the last 1/4 of the first rope burns, half of the second rope burns. This will bring the time to 60 second. Half of the second rope will still be left but since both are burning at the same time it will only take 15 min to burn and will burn out simultaneously. This will bring the total to 75min or 1:15 (if i understood correctly and assuming that you tie the ropes perfectly)

[martyr]

numenor: wouldn't work, since part two's ropes don't burn linearly. there's no way to be sure that the 75% mark is equal to 45min (or 50% = 30m)

[Rahl]

Dip the ropes in urine to reduce burn rate. Problem solved.

[martyr]

given that rahl's urine is a uniform fire retardant (omgffsrobot) that still won't make the ropes burn linearly...

[Numenor]

Crap ... I thought that part was a joke

[elana]

actually, there is a solution to the non-linear ropes, and it doesn't require any additional rope or torture.

Who said anything about it requiring torture. It's just more fun that way :)