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Probablity

by cmal_s » Thu Mar 25, 2010 12:17 am
A contest consists of n questions, each can be answered as True or False. Anyone who ansers all n questions correctly will b e a winner. what is the least value of n for which the probablity will is less that 1/1000 that a person who randomly guesses the answer to each question will be a winner


a 5
b 10
c 50
d 100
e 1000
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by eaakbari » Thu Mar 25, 2010 12:36 am
Could you please give OA.

Probability of answering each question correctly is 1/2.

Hence answering n questions correctly is 1/2^n

According to the condition 1/2^n < 1000
so 2^n > 1000
Proceed by picking values and a little sense of approximation

For the probability to be less than 1/1000 then n = 10 (2^n = 1024), being answer choice b


E
Last edited by eaakbari on Thu Mar 25, 2010 5:03 am, edited 1 time in total.
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by cmal_s » Thu Mar 25, 2010 12:43 am
whats OA?

alot of people are asking
the correct Answer is 10?
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by cmal_s » Thu Mar 25, 2010 12:46 am
i looked at the question again and it says Probablity less than 1/1000
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by eaakbari » Thu Mar 25, 2010 12:51 am
OA refers to the actual answer. Do not know what exactly it stands for.
Last edited by eaakbari on Thu Mar 25, 2010 5:04 am, edited 1 time in total.
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by harshavardhanc » Thu Mar 25, 2010 12:59 am
cmal_s wrote:A contest consists of n questions, each can be answered as True or False. Anyone who ansers all n questions correctly will b e a winner. what is the least value of n for which the probablity will is less that 1/1000 that a person who randomly guesses the answer to each question will be a winner


a 5
b 10
c 50
d 100
e 1000
cmal_s,

I don't know if you are doing this deliberately, but why are you posting PS questions in DS forum and vise-versa? There should be some reason behind it. correct? would surely like to know it......
Regards,
Harsha
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by thephoenix » Thu Mar 25, 2010 2:24 am
cmal_s wrote:A contest consists of n questions, each can be answered as True or False. Anyone who ansers all n questions correctly will b e a winner. what is the least value of n for which the probablity will is less that 1/1000 that a person who randomly guesses the answer to each question will be a winner


a 5
b 10
c 50
d 100
e 1000
probaibilty of true or false=1/2
if n no. of questions are there then probability=1/2^n<1/1000
we need to find least value of n
n=9---->2^9=512
n=10---->2^n=1024
and 1/1024<1/1000
hence n=10
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by November Rain » Thu Mar 25, 2010 3:10 am
hi,
Using exponent properties, you can quickly come up with the answer.

The probability of randomly getting a right answer on a true or false question is (1/2). If you have two questions the probability would be (1/2)*(1/2)

Now, if the probability needs to be below 1/1000, then (1/2)^N < (1/1000), or (1/2)^N < (1/2)^3 * (1/5)^3

Since you now that (1/5)^3 is about (1/125), we need to find a value of N such that:

(1/2)^N < (1/2)^3 * (1/125). Since you now that 2^7 =128 The closest you have of (1/1000) (altough still below this value) is

(1/2)^3 * (1/2)^7

So N = 10
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by Haldiram Bhujiawala » Thu Mar 25, 2010 3:15 am
IMO [spoiler]b) 10[/spoiler]
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by sanju09 » Thu Mar 25, 2010 5:13 am
cmal_s wrote:A contest consists of n questions, each can be answered as True or False. Anyone who ansers all n questions correctly will b e a winner. what is the least value of n for which the probablity will is less that 1/1000 that a person who randomly guesses the answer to each question will be a winner


a 5
b 10
c 50
d 100
e 1000
On many occassions, a valid question appears to be ambiguous owing to typo, please check everything/word placed properly before posting. This is a PS question, and the forum has a separate sub-forum for it. [spoiler]I hope the moderators are alert[/spoiler]. Answer to the question is by all means [spoiler]10[/spoiler], if what I perceived is correct.
The mind is everything. What you think you become. -Lord Buddha



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by el_torero » Fri Mar 26, 2010 1:28 pm
i nice rule to use here that i picked up in someone else's post is that 2^10 = "binary 1000" = 1024, an easy way to remember a huge power of 2. this helps you deduce what n is really quickly.
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