A few DS from GMATPREP

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ua: Newbie | Next Rank: 10 Posts; Posts: 8; Joined: Sat Nov 18, 2006 3:24 am

A few DS from GMATPREP

by ua » Sun Nov 18, 2007 3:12 am

1. Is x < y
(a) 2x < 3y
(b) xy > 0

2. An integer greater than 1 that is not a prime is called composite. If a two-digit integer n is greater than 20, is n composite?
(a) The tens digit of n is a factor of the units digit of n
(b) The tens digit of n is 2

3. The attendees at a certain convention purchased a total of 15,000 books. How many of these attendees were female?
(a) There were total of 4,000 attendees at the convention
(b) The male attendees purchased an average of 3 books each and females attendees purchased an average of 5 books each.

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Source: Beat The GMAT — Data Sufficiency | Sun Nov 18, 2007 3:12 am

gabriel: Legendary Member; Posts: 986; Joined: Wed Dec 20, 2006 11:07 am; Location: India; Thanked: 51 times; Followed by:1 members

by gabriel » Sun Nov 18, 2007 5:18 am

In the future, post one question per thread .

Regards

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ri2007: Master | Next Rank: 500 Posts; Posts: 258; Joined: Mon Aug 27, 2007 12:43 pm; Thanked: 15 times

Re: A few DS from GMATPREP

by ri2007 » Sun Nov 18, 2007 5:34 am

ua wrote:1. Is x < y
(a) 2x <3y> 0

2. An integer greater than 1 that is not a prime is called composite. If a two-digit integer n is greater than 20, is n composite?
(a) The tens digit of n is a factor of the units digit of n
(b) The tens digit of n is 2

3. The attendees at a certain convention purchased a total of 15,000 books. How many of these attendees were female?
(a) There were total of 4,000 attendees at the convention
(b) The male attendees purchased an average of 3 books each and females attendees purchased an average of 5 books each.

Ans 1) E
Question X<Y
1) X<1>20 is n a prime no?

Statement 1) tens digit of n is a factor of the units digit. Such numbers above 20 cannot be prime. Try it out before the exam

2) may or may not be prime e.g 29 or 23 are prime, 27 is not

Ans 3) C

1) M+F = 4000
2) 3M + 2F = 15000

Together

3M + 2 (4000 - M) = 15000

Please confirm the ans.

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ua: Newbie | Next Rank: 10 Posts; Posts: 8; Joined: Sat Nov 18, 2006 3:24 am

by ua » Sun Nov 18, 2007 2:28 pm

Gabriel: I apologize.

1. Answer is E

Just wanted to understand how the first statement is not enough?

2x < 3y

=> x <3y/2

Doesn't this give a relationship enough to prove x < y.
I know its wrong but I cannot seem to prove it.

2. Answer is A

Statement 1: First of all there are infinite number of primes greater than 20. So there should be a mathematical rule which concludes whether this statement is correct or not. Because one cannot plug-in all the way.

Secondly, if I understand the statement correctly, then 139 should not be a prime.

Statement 2: Its simple to evaluate

3. Answer is C
I think my mind blanked-out on this one. I ended up concluding that an average of 5 cannot be reached by a single number. For instance [3,4,5,6,7] and [3,5,7]. And this forced thought to ignore the ratio in statement 1.

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ri2007: Master | Next Rank: 500 Posts; Posts: 258; Joined: Mon Aug 27, 2007 12:43 pm; Thanked: 15 times

by ri2007 » Sun Nov 18, 2007 2:36 pm

I just re- read my post and looks like some of it was erased before i actually posted it.

Any way

Q1) x < 3/2 Y ...3/2 = 1.5 so X can be equal to , greater or even less than y

Q2) The question states it is a 2 digit integer. So there is not an endless no. of integers. Just use logis, if in a 2 digit integer the 10th digit is a factor of the units digit, then the tens digit is fully divisible by the units digit, also the units digit is fully divisible by the units digit. So how can it be prime?