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simplyjat: Master | Next Rank: 500 Posts; Posts: 423; Joined: Thu Dec 27, 2007 1:29 am; Location: Hyderabad, India; Thanked: 36 times; Followed by:2 members; GMAT Score:770

MGMAT question.

by simplyjat » Wed Apr 02, 2008 9:04 am

This is a question for mgmat. For which I do not agree with the explanation. What do you guys say...

Is z even?
(1) z/2 is even.
(2) 3z is even.

Moderator, if it is not OK to post mgat questions here, kindly delete the post.

simplyjat

Quote

Source: Beat The GMAT — Data Sufficiency | Wed Apr 02, 2008 9:04 am

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: MGMAT question.

by Stuart@KaplanGMAT » Wed Apr 02, 2008 11:32 am

simplyjat wrote:This is a question for mgmat. For which I do not agree with the explanation. What do you guys say...

Is z even?
(1) z/2 is even.
(2) 3z is even.

Moderator, if it is not OK to post mgat questions here, kindly delete the post.

(1) z/2 = even

Like any other equation, we can manipulate this to:

z = 2*even

Well, 2 times any integer will be even, therefore z will always be even: sufficient.

(2) 3z = even.

z = even/3

Will all even numbers divided by 3 be even? No. Will some of them be even? yes.

For example, we could have picked:

z = 6/3 = 2. Is 2 even? Yes.

or

z = 4/3 = 1.333. Is 1.333 even? No.

Since we can get a "yes" and a "no" answer, (2) is insufficient.

(1) is sufficient, (2) is insufficient: choose (A).

* * *

If you chose (D), both sufficient, you probably made the erroneous assumption that z had to be an integer. However, since we're never told that z must be an integer, we can't preclude other possibilities.