# interview question for IBWatch

#1
my brother went for an interview 3rd round with goldman and dey askd him dis question (derivatives) if you have 50 red balls and 50 black balls and two jars to put them in what is de best way to put them into two jars so that you will definately pick a red? (its not 50-50)
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13 years ago
#2
(Original post by talentless)
my brother went for an interview 3rd round with goldman and dey askd him dis question (derivatives) if you have 50 red balls and 50 black balls and two jars to put them in what is de best way to put them into two jars so that you will definately pick a red? (its not 50-50)
Scrap the black ones ?
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13 years ago
#3
well, the solution to the problem (maximising expectation of getting a red ball, while using all 100) is to put one red ball in one of the jars, and the other 99 balls in the other jar, giving a probability of

0.5 + 0.5*(49/99) = 0.7474....

approximately 75%

My favourite question of this type:
Assuming the Earth is a perfect sphere, describe all the points where the walk one mile south, one mile east and then one mile north brings you to exactly the same place.
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13 years ago
#4
^ im not even understanding the question :/
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13 years ago
#5
(Original post by Olek)
well, the solution to the problem (maximising expectation of getting a red ball, while using all 100) is to put one red ball in one of the jars, and the other 99 balls in the other jar, giving a probability of

0.5 + 0.5*(49/99) = 0.7474....

approximately 75%

My favourite question of this type:
Assuming the Earth is a perfect sphere, describe all the points where the walk one mile south, one mile east and then one mile north brings you to exactly the same place.
Shouldn't it be ?
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13 years ago
#6
(Original post by Knogle)
Shouldn't it be ?
Umm dude from what I've done in probability - what do you call those ? - (a fair bit of this year's math programme), I'd say Olek's right.
0.5(since that's the chance of picking that jar)*1 + 0.5*(the 49 other red balls out of the 100-1).
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13 years ago
#7
(Original post by Johan C)
Umm dude from what I've done in probability - what do you call those ? - (a fair bit of this year's math programme), I'd say Olek's right.
0.5(since that's the chance of picking that jar)*1 + 0.5*(the 49 other red balls out of the 100-1).
Ah, apologies. I misread the question.
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13 years ago
#8
(Original post by Knogle)
Ah, apologies. I misread the question.
Lol it happens to me all the time in those stupid logic tests
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13 years ago
#9
Olek - the north and south pole.

Olek's answer on the balls is correct. I remember it being asked on ibtalk and someone going through the process of differentiating a function to find the maximum and minimum points etc etc before I came up with the answer similar to the above
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13 years ago
#10
(Original post by President_Ben)
Olek - the north and south pole.

Olek's answer on the balls is correct. I remember it being asked on ibtalk and someone going through the process of differentiating a function to find the maximum and minimum points etc etc before I came up with the answer similar to the above

From your experience, are these sorts of questions common at interviews? Or is it a matter of being unlucky if you get asked these?
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13 years ago
#11
The differentiation that person did was wrong (if I remember correctly). I tried to correct it and failed (I think I had some work to be doing at the time)...

Anyway, Ben, there are more points, the south pole is the degenerate case, the north pole the exception. Took me ages to work it out.
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13 years ago
#12
I'm an economist, not a mathemo/physicist/engineer. I still operate on euclidean geometry

To find two of the points is a miracle What are the others?

I'm pretty sure the above question can be solved differentiation. It certainly has a nice rates of change action going on... but if you are trying to solve it with differentiation, you're on the wrong track.
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13 years ago
#13
(Original post by slick_rick)
From your experience, are these sorts of questions common at interviews? Or is it a matter of being unlucky if you get asked these?
Expect at least one... if you're good, you'll work it out and it'll show. If you're not so good, well, it's only one question... but many of these questions are regularly occuring so it can sort of become a learning exercise where you end up trying to convince someone you don't know the answer.

The angle between... 7.5!
A waterlily... 58 minutes!
On a running track... impossible!
A cube... 512!

etc.
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13 years ago
#14
I've never heard the running track one... what is the question?

On further consideration, I've remembered the south pole doesn't work, because you walk north as the last step, which is defined and takes you away from the south pole.

With the walk: consider a circle around the south pole, such that the circumference is exactly one mile, any point one mile north of any point on this circle satisfies the locus (you walk south one mile, onto this circle, walk east one mile around it, arriving exactly where you were, then walk one mile north again, back to the same spot). Now consider all circles such that the circumference is exactly 1/n miles for all natural numbers n, the points one mile north of all these circles all satisfy the locus.
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13 years ago
#15
(Original post by talentless)
my brother went for an interview 3rd round with goldman and dey askd him dis question (derivatives) if you have 50 red balls and 50 black balls and two jars to put them in what is de best way to put them into two jars so that you will definately pick a red? (its not 50-50)

assuming that 50 balls would fill a jar completely, then:

start with 25 black balls, and then on top of them, put 25 red balls. Do the same in the other jar.
0
13 years ago
#16
my brother went for an interview 3rd round with goldman and dey askd him dis question (derivatives) if you have 50 red balls and 50 black balls and two jars to put them in what is de best way to put them into two jars so that you will definately pick a red? (its not 50-50)
Are you sure the question wasn't "if you have...the best way to put them into two jars whilst blind folded.....pick red"?
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13 years ago
#17
(Original post by TheAsianProdigy)

assuming that 50 balls would fill a jar completely, then:

start with 25 black balls, and then on top of them, put 25 red balls. Do the same in the other jar.
you can't answer a question by redefining the situation. Imagine how nice maths exams would be if you could...
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13 years ago
#18
I actually like these problems and it would give me great pleasure to maybe have one to solve when I log in here now and then (life has taken a slow pace recently). It would be a good resource for others who may be going for interviews and want/need to cheat on these questions by having the solutions to the common ones memorised. Perhaps a list could be set up here.
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13 years ago
#19
(Original post by TheAsianProdigy)

assuming that 50 balls would fill a jar completely, then:

start with 25 black balls, and then on top of them, put 25 red balls. Do the same in the other jar.
Bruv, thats what i thought. Great minds think alike!
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13 years ago
#20
(Original post by coughsyrup)
you can't answer a question by redefining the situation. Imagine how nice maths exams would be if you could...
But have we not assumed that we can put 99 balls in one jar? Why not 50 then?

Im just trying to get my head round this.
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