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A test maker is to design a probability test from a list of

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A test maker is to design a probability test from a list of [#permalink] New post 07 Oct 2009, 05:57
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A
B
C
D
E

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A test maker is to design a probability test from a list of 21 questions. The 21 questions are classified into three categories: hard, intermediate and easy. If there are 7 questions in each category and the test maker is to select two questions from each category, how many different combinations of questions can the test maker put on the test?

A) 3*(7!/(5!2!))
B) 3*(7!/(5!))
C) 3*(7!/(2!))
D) 3*21
E) 72
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Re: Test maker - combinations problem [#permalink] New post 07 Oct 2009, 06:41
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mendelay wrote:
A test maker is to design a probability test from a list of 21 questions. The 21 questions are classified into three categories: hard, intermediate and easy. If there are 7 questions in each category and the test maker is to select two questions from each category, how many different combinations of questions can the test maker put on the test?
A) 3*(7!/(5!2!))
B) 3*(7!/(5!))
C) 3*(7!/(2!))
D) 3*21
E) 72


There is no correct choice!

The number of combinations will be C(2,7)*C(2,7)*C(2,7)=C(2,7)^3=21^3=9261

Check the source. A and C give the same answers.

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Re: Test maker - combinations problem [#permalink] New post 07 Oct 2009, 06:59
The question is from PlatinumGmat http://www.platinumgmat.com/practice_gm ... on_id=2252
and the OA given there is A.

However, I disagree because 7C2*7C2*7C2 ≠ 37C2.
I think the answer is none of the above.
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Re: Test maker - combinations problem [#permalink] New post 07 Oct 2009, 07:05
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mendelay wrote:
The question is from PlatinumGmat http://www.platinumgmat.com/practice_gm ... on_id=2252
and the OA given there is A.

However, I disagree because 7C2*7C2*7C2 ≠ 37C2.
I think the answer is none of the above.


They made a mistake: 7C2*7C2*7C2 ≠ 37C2 its (7C2)^3=9261. There must be a typo.

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NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: Test maker - combinations problem [#permalink] New post 15 Nov 2013, 05:06
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Re: Test maker - combinations problem   [#permalink] 15 Nov 2013, 05:06
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