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Help me understand this one pls...

Author Message
Manager
Joined: 26 Mar 2010
Posts: 125
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Help me understand this one pls... [#permalink]

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17 Apr 2010, 00:42
00:00

Difficulty:

(N/A)

Question Stats:

100% (01:52) correct 0% (00:00) wrong based on 3 sessions

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If 75% of a class answered the 1st question on a certain test correctly, 55% answered the 2nd question on the test correctly and 20% answered neither of the questions correctly, what percent answered both correctly?
10%
20%
30%
50%
65%
Manager
Joined: 13 Dec 2009
Posts: 129
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Re: Help me understand this one pls... [#permalink]

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17 Apr 2010, 05:38
utin wrote:
If 75% of a class answered the 1st question on a certain test correctly, 55% answered the 2nd question on the test correctly and 20% answered neither of the questions correctly, what percent answered both correctly?
10%
20%
30%
50%
65%

from the question stem: 20% answered neither of the two => 80% answered both the questions.
so, we have
Q1or Q2 = Q1+Q2- Q1&Q2 => Q1&Q2 = Q1+Q2-Q1orQ2
=> Q1&Q2 = 75+55-80 = 50%

Intern
Joined: 21 Dec 2009
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Re: Help me understand this one pls... [#permalink]

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17 Apr 2010, 05:47
Formula:
Total = A + B + None - Both

So,

100 = 75 + 55 + 20 - Both
Both = 50%

Thanks
BSchool Forum Moderator
Joined: 01 Oct 2009
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Re: Help me understand this one pls... [#permalink]

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25 Apr 2010, 08:55
Manager
Joined: 27 Dec 2009
Posts: 171
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Re: Help me understand this one pls... [#permalink]

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25 Apr 2010, 09:01
yes ... fairly easy.
Re: Help me understand this one pls...   [#permalink] 25 Apr 2010, 09:01
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