Section 1 / Q 20 in 1000PS

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priyasom: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Tue May 29, 2007 10:36 pm

Section 1 / Q 20 in 1000PS

by priyasom » Thu Jan 31, 2008 7:46 pm

If y ≠ 3 and 3x/y is a prime integer greater than 2, which of the following must be true?
I. x = y
II. y = 1
III x and y are prime integers.

(A) None
(B) I only
(C) II only
(D) III only
(E) I and III

OA is (A)
Can someone explain why I ( x= y) is not true?

Thanks

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Source: Beat The GMAT — Problem Solving | Thu Jan 31, 2008 7:46 pm

Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pm; Location: Toronto; Thanked: 1710 times; Followed by:614 members; GMAT Score:800

Re: Section 1 / Q 20 in 1000PS

by Stuart@KaplanGMAT » Thu Jan 31, 2008 8:02 pm

priyasom wrote:If y ≠ 3 and 3x/y is a prime integer greater than 2, which of the following must be true?
I. x = y
II. y = 1
III x and y are prime integers.

(A) None
(B) I only
(C) II only
(D) III only
(E) I and III

OA is (A)
Can someone explain why I ( x= y) is not true?

Thanks

If we pick x= -5 and y= -3, we get 3(-5)/(-3) = 5. So, x doesn't have to equal y.

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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wawatan: Master | Next Rank: 500 Posts; Posts: 171; Joined: Sat May 03, 2008 3:51 pm; Thanked: 8 times

by wawatan » Tue May 20, 2008 5:24 pm

What is wrong with option III? anyone? how do you plug in numbers for this?

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netigen: Legendary Member; Posts: 631; Joined: Mon Feb 18, 2008 11:57 pm; Thanked: 29 times; Followed by:3 members

by netigen » Tue May 20, 2008 10:16 pm

x and y can be both negative for e.g. x=-7 and y=-3 hence not prime

or x and y can be both equal for e.g. x=4 and y=4 and may or may not be prime

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wawatan: Master | Next Rank: 500 Posts; Posts: 171; Joined: Sat May 03, 2008 3:51 pm; Thanked: 8 times

by wawatan » Wed May 21, 2008 12:52 am

thanks! for some reason, i forgot that prime numbers are positive numbers...but i looked it up on wikipedia. great explanation. also for plugging in numbers if the question says x and y ....then x can equal y when you plug in numbers? or you have to plug in different numbers because its different letters?

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by VP_RedSoxFan » Wed May 21, 2008 2:18 am

From what I can tell, to make 3x/y prime and have y not equal to 3, y has to be either x, 1 or -3. The allowance for x and y (at the same time, only) to be negative is what makes III not necessarily true.

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