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After taking N tests, each containing 100 questions

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After taking N tests, each containing 100 questions  [#permalink]

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New post 11 Jun 2013, 13:04
1
00:00
A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

73% (02:04) correct 27% (02:32) wrong based on 98 sessions

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After taking N tests, each containing 100 questions, John had an average of 70% of correct answers. How much does John need to score on the next test to make his average equal 72%?

A. N−35
B. N+72
C. 2N+70
D. 2N+72
E. 2N−35

M13-03.
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Re: After taking N tests, each containing 100 questions  [#permalink]

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New post 11 Jun 2013, 13:14
2
gmatquant25 wrote:
After taking N tests, each containing 100 questions, John had an average of 70% of correct answers. How much does John need to score on the next test to make his average equal 72%?

A. N−35
B. N+72
C. 2N+70
D. 2N+72
E. 2N−35


Say N=1.

So, after 1 test John has 70 correct answers.
In 2 tests, so in 200 questions he needs to have 0.72*200=144 correct answers, so in the second test he must get 144-70=74 questions correctly.

Now, plug N=1 into the answer choices to see which one yields 74. Only option D fits.

Answer: D.

Is this question from GMAT Club tests? If yes, can you please tell the test and question numbers? Thank you.
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Re: After taking N tests, each containing 100 questions  [#permalink]

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New post 11 Jun 2013, 13:19
gmatquant25 wrote:
After taking N tests, each containing 100 questions, John had an average of 70% of correct answers. How much does John need to score on the next test to make his average equal 72%?

A. N−35
B. N+72
C. 2N+70
D. 2N+72
E. 2N−35


Answer is [D].

Total right answers before the final test = 70N
Total right answers after last test = 72/100 * (100*(N+1)) = 72N + 72

Hence total right answers in the last test = 72N + 72 - 70N = 2N + 72.

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Re: After taking N tests, each containing 100 questions  [#permalink]

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New post 12 Jun 2013, 00:01
1
gmatquant25 wrote:
After taking N tests, each containing 100 questions, John had an average of 70% of correct answers. How much does John need to score on the next test to make his average equal 72%?

A. N−35
B. N+72
C. 2N+70
D. 2N+72
E. 2N−35

M13-03.


I. After N tests, John did 100N question, 70% correct = 70N question correct.
II. After (N + 1) tests, John did 100(N+1) question, 72% correct = 72(N + 1)
III. However, total correct question = 70N + x.

==> connect II and III ==> 72(N + 1) = 70N + x
==> x = 72 + 2N

C is correct.
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Re: After taking N tests, each containing 100 questions  [#permalink]

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New post 12 Jun 2013, 02:15
1
gmatquant25 wrote:
After taking N tests, each containing 100 questions, John had an average of 70% of correct answers. How much does John need to score on the next test to make his average equal 72%?

A. N−35
B. N+72
C. 2N+70
D. 2N+72
E. 2N−35


Just another approach :

We can safely assume that John got 70 correct answers in each of the N tests. Thus, to make this to 72 correct answers across, the new score should be such that it contributes 2 correct answers to each of the N test, and after this, is left with an exact score of 72, thus making the new average for the (N+1) tests as 72--> 2*N+72.

D.
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Re: After taking N tests, each containing 100 questions  [#permalink]

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New post 09 Jul 2015, 11:24
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: After taking N tests, each containing 100 questions   [#permalink] 09 Jul 2015, 11:24
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