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examination

by shashank.ism » Tue Feb 09, 2010 1:05 pm
In a certain examination paper, there are n questions. For j = 1, 2 ...n, there are 2^(n-j) students who answered j or more questions wrongly. If the total number of wrong answers is 4095, then the value of n is

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by harsh.champ » Wed Feb 10, 2010 11:49 pm
shashank.ism wrote:In a certain examination paper, there are n questions. For j = 1, 2 ...n, there are 2^(n-j) students who answered j or more questions wrongly. If the total number of wrong answers is 4095, then the value of n is

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Let us say there are only 3 questions. Thus there are 23-1 = 4 students who have done 1 or more
questions wrongly, 23-2 = 2 students who have done 2 or more questions wrongly and 23-3 = 1 student who
must have done all 3 wrongly. Thus total number of wrong answers = 4 + 2 + 1 = 7 = 23 - 1 = 2n - 1.
In our question, the total number of wrong answers = 4095 = 212 - 1. Thus [spoiler]n = 12.A is the ans.[/spoiler]
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by odod » Thu Feb 11, 2010 12:27 pm
Let us say there are only 3 questions. Thus there are 23-1 = 4 students who have done 1 or more
questions wrongly, 23-2 = 2 students who have done 2 or more questions wrongly and 23-3 = 1 student who
must have done all 3 wrongly. Thus total number of wrong answers = 4 + 2 + 1 = 7 = 23 - 1 = 2n - 1.
In our question, the total number of wrong answers = 4095 = 212 - 1. Thus n = 12.A is the ans.
_________________

I'm having trouble understanding this "Thus there are 23-1 = 4 students who have done 1 or more
questions wrongly"

Where is the 23-1=4 coming from?
ODOD
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by ajith » Thu Feb 11, 2010 12:42 pm
odod wrote:Let us say there are only 3 questions. Thus there are 23-1 = 4 students who have done 1 or more
questions wrongly, 23-2 = 2 students who have done 2 or more questions wrongly and 23-3 = 1 student who
must have done all 3 wrongly. Thus total number of wrong answers = 4 + 2 + 1 = 7 = 23 - 1 = 2n - 1.
In our question, the total number of wrong answers = 4095 = 212 - 1. Thus n = 12.A is the ans.
_________________

I'm having trouble understanding this "Thus there are 23-1 = 4 students who have done 1 or more
questions wrongly"

Where is the 23-1=4 coming from?
2^(3-1) is what the author means which is indeed 2^2 = 4. Probably the answer was copied from somewhere and the power sign was missed
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by Osirus@VeritasPrep » Thu Feb 11, 2010 12:43 pm
ajith wrote:
odod wrote:Let us say there are only 3 questions. Thus there are 23-1 = 4 students who have done 1 or more
questions wrongly, 23-2 = 2 students who have done 2 or more questions wrongly and 23-3 = 1 student who
must have done all 3 wrongly. Thus total number of wrong answers = 4 + 2 + 1 = 7 = 23 - 1 = 2n - 1.
In our question, the total number of wrong answers = 4095 = 212 - 1. Thus n = 12.A is the ans.
_________________

I'm having trouble understanding this "Thus there are 23-1 = 4 students who have done 1 or more
questions wrongly"

Where is the 23-1=4 coming from?
2^(3-1) is what the author means which is indeed 2^2 = 4. Probably the answer was copied from somewhere and the power sign was missed
yes, it was copied from www.complore.com

That is their scam, they just copy and paste questions and then the other copies and paste the solution.
https://www.beatthegmat.com/the-retake-o ... 51414.html

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