It is currently 13 Dec 2017, 20:48

# Decision(s) Day!:

CHAT Rooms | Ross R1 | Kellogg R1 | Darden R1 | Tepper R1

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# If x and y are positive integer and xy is divisible by 4

Author Message
TAGS:

### Hide Tags

Manager
Joined: 06 May 2012
Posts: 75

Kudos [?]: 57 [0], given: 16

If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

05 Nov 2012, 17:18
2
This post was
BOOKMARKED
00:00

Difficulty:

15% (low)

Question Stats:

72% (00:59) correct 28% (00:48) wrong based on 160 sessions

### HideShow timer Statistics

If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?

A) If $$x$$ is even then $$y$$ is odd.
B) If $$x$$ is odd then $$y$$ is a multiple of 4.
C) If $$x+y$$ is odd then $$y/x$$ is not an integer.
D) If $$x+y$$ is even then $$x/y$$ is an integer.
E) $$x^y$$ is even.

[Reveal] Spoiler: OA

Kudos [?]: 57 [0], given: 16

Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 4554

Kudos [?]: 8959 [3], given: 111

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

05 Nov 2012, 18:14
3
KUDOS
Expert's post
gmatchase wrote:
If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?
A) If $$x$$ is even then $$y$$ is odd.
B) If $$x$$ is odd then $$y$$ is a multiple of 4.
C) If $$x+y$$ is odd then $$y/x$$ is not an integer.
D) If $$x+y$$ is even then $$x/y$$ is an integer.
E) $$x^y$$ is even.

I'm happy to help with this.

First of all, this is a very challenging GMAT problem --- it really demands a great deal of number sense to answer this efficiently.

The prompt tells us x & y are positive integer (mercifully!) and x*y is divisible by 4. That could happen if either one of them is a multiple of four, or if both of them are even.

(A) Counter-example: If x = 2, then y could not be odd, because 2*(odd) will never be divisible by four. NOT ALWAYS TRUE.
(B) If x is odd, then there are no factors of 2 in x. There are two factors of 2 in x*y, since x*y is be divisible by 4. If those two factors of 2 don't come from x, they must come from y. Therefore y MUST BE divisible by four.
(C) Counter-example: x = 3 and y = 12 --- x + 12 = 15 is odd, but y/x = 12/3 = 4 is an integer. NOT ALWAYS TRUE.
(D) Counter-example: x = 12 and y = 8 ---- x + y = 20 is even, but x/y = 12/8 = 3/2 is not an integer. NOT ALWAYS TRUE.
(E) Counter-example: x = 3 and y = 4 ---- x^y = 3^4 = 81 is not even. NOT ALWAYS TRUE.

In every pair in the counter-examples, the numbers were chosen so that x*y is divisible by 4, and any additional condition in that particular choice is met.

This blog is tangentially related -- it may help a bit with number sense:
http://magoosh.com/gmat/2012/gmat-math- ... variables/

Does all this make sense?

Mike
_________________

Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)

Kudos [?]: 8959 [3], given: 111

Manager
Joined: 06 May 2012
Posts: 75

Kudos [?]: 57 [0], given: 16

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

05 Nov 2012, 19:41
Quote:
If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?

Thank you for the reply Mike. When I saw $$xy$$, I didn't think that it is $$x*y$$. I thought it might be below

$$x$$ --> 1
$$y$$ --> 4

then $$xy$$ is 14. Is this not the case? If not, can you explain when the expression indicates $$x*y$$?

Kudos [?]: 57 [0], given: 16

Math Expert
Joined: 02 Sep 2009
Posts: 42598

Kudos [?]: 135563 [0], given: 12699

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

06 Nov 2012, 02:39
gmatchase wrote:
Quote:
If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?

Thank you for the reply Mike. When I saw $$xy$$, I didn't think that it is $$x*y$$. I thought it might be below

$$x$$ --> 1
$$y$$ --> 4

then $$xy$$ is 14. Is this not the case? If not, can you explain when the expression indicates $$x*y$$?

If xy meant to be two-digit integer xy, then it would be mentioned something like "x and y are digits of two-digit integer xy".

If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?

A. If $$x$$ is even then $$y$$ is odd.
B. If $$x$$ is odd then $$y$$ is a multiple of 4.
C. If $$x+y$$ is odd then $$\frac{y}{x}$$ is not an integer.
D. If $$x+y$$ is even then $$\frac{x}{y}$$ is an integer.
E. $$x^y$$ is even.

Notice that the question asks which of the following MUST be true not COULD be true.

A. If $$x$$ is even then $$y$$ is odd --> not necessarily true, consider: $$x=y=2=even$$;

B. If $$x$$ is odd then $$y$$ is a multiple of 4 --> always true: if $$x=odd$$ then in order $$xy$$ to be a multiple of 4 y mst be a multiple of 4;

C. If $$x+y$$ is odd then $$\frac{y}{x}$$ is not an integer --> not necessarily true, consider: $$x=1$$ and $$y=4$$;

D. If $$x+y$$ is even then $$\frac{x}{y}$$ is an integer --> not necessarily true, consider: $$x=2$$ and $$y=4$$;

E. $$x^y$$ is even --> not necessarily true, consider: $$x=1$$ and $$y=4$$;

_________________

Kudos [?]: 135563 [0], given: 12699

Manager
Joined: 06 May 2012
Posts: 75

Kudos [?]: 57 [0], given: 16

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

06 Nov 2012, 04:33
Thank you for your explanation Bunuel

Kudos [?]: 57 [0], given: 16

Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 4554

Kudos [?]: 8959 [1], given: 111

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

06 Nov 2012, 09:54
1
KUDOS
Expert's post
gmatchase wrote:
Thank you for the reply Mike. When I saw xy, I didn't think that it is x*y. I thought it might be below

x --> 1
y --> 4

then xy is 14. Is this not the case? If not, can you explain when the expression indicates x*y?

Dear gmatchase

First of all, I'd like to suggest --- don't over-do the "math" notation. That's useful for fractions & exponents, but for ordinary x+y, we don't need the special notation.

The blanket assumption throughout all of algebra is that writing two variables next to each other (mx) or writing a number directly next to a variable (5y) always means multiplication. That's the meaning 100% of the time, unless something different is very explicitly specified. If the problem wanted the variables to represent digits, first of all, it probably would have used letters from the beginning of the alphabet, certainly not x & y (the standard algebraic variables), and it would have to say, quite explicitly, something like "Let a and b be digits in a the two-digit number ab." In other words, because the blank assumption is --- two variables next to each other always means multiply --- when a problem wants to indicate digits or anything else, it practically has to set up traffic cones and flashing signs to indicate the change. For a digits problem, it would have to explain, explicitly, what each variable represented. Any problem that just tosses an xy at you with no verbiage will never be using those variable as digits --- that xy will be multiplication 100% of the time.

Here's an example of a rare digit problem.
http://gmat.magoosh.com/questions/54
When you submit you answer, the following page will have a full video explanation.

Does all this make sense?

Mike
_________________

Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)

Kudos [?]: 8959 [1], given: 111

Manager
Joined: 06 May 2012
Posts: 75

Kudos [?]: 57 [0], given: 16

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

06 Nov 2012, 15:18
mikemcgarry wrote:
gmatchase wrote:
Thank you for the reply Mike. When I saw xy, I didn't think that it is x*y. I thought it might be below

x --> 1
y --> 4

then xy is 14. Is this not the case? If not, can you explain when the expression indicates x*y?

Dear gmatchase

First of all, I'd like to suggest --- don't over-do the "math" notation. That's useful for fractions & exponents, but for ordinary x+y, we don't need the special notation.

The blanket assumption throughout all of algebra is that writing two variables next to each other (mx) or writing a number directly next to a variable (5y) always means multiplication. That's the meaning 100% of the time, unless something different is very explicitly specified. If the problem wanted the variables to represent digits, first of all, it probably would have used letters from the beginning of the alphabet, certainly not x & y (the standard algebraic variables), and it would have to say, quite explicitly, something like "Let a and b be digits in a the two-digit number ab." In other words, because the blank assumption is --- two variables next to each other always means multiply --- when a problem wants to indicate digits or anything else, it practically has to set up traffic cones and flashing signs to indicate the change. For a digits problem, it would have to explain, explicitly, what each variable represented. Any problem that just tosses an xy at you with no verbiage will never be using those variable as digits --- that xy will be multiplication 100% of the time.

Here's an example of a rare digit problem.
http://gmat.magoosh.com/questions/54
When you submit you answer, the following page will have a full video explanation.

Does all this make sense?

Mike

Thank you again for all your explanation. It makes sense now.

Kudos [?]: 57 [0], given: 16

Director
Joined: 27 Jun 2011
Posts: 879

Kudos [?]: 247 [0], given: 57

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

25 Jun 2013, 08:23
Bunuel wrote:
gmatchase wrote:
Quote:
If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?

Thank you for the reply Mike. When I saw $$xy$$, I didn't think that it is $$x*y$$. I thought it might be below

$$x$$ --> 1
$$y$$ --> 4

then $$xy$$ is 14. Is this not the case? If not, can you explain when the expression indicates $$x*y$$?

If xy meant to be two-digit integer xy, then it would be mentioned something like "x and y are digits of two-digit integer xy".

If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?

A. If $$x$$ is even then $$y$$ is odd.
B. If $$x$$ is odd then $$y$$ is a multiple of 4.
C. If $$x+y$$ is odd then $$\frac{y}{x}$$ is not an integer.
D. If $$x+y$$ is even then $$\frac{x}{y}$$ is an integer.
E. $$x^y$$ is even.

Notice that the question asks which of the following MUST be true not COULD be true.

A. If $$x$$ is even then $$y$$ is odd --> not necessarily true, consider: $$x=y=2=even$$;

B. If $$x$$ is odd then $$y$$ is a multiple of 4 --> always true: if $$x=odd$$ then in order $$xy$$ to be a multiple of 4 y mst be a multiple of 4;

C. If $$x+y$$ is odd then $$\frac{y}{x}$$ is not an integer --> not necessarily true, consider: $$x=1$$ and $$y=4$$;

D. If $$x+y$$ is even then $$\frac{x}{y}$$ is an integer --> not necessarily true, consider: $$x=2$$ and $$y=4$$;

E. $$x^y$$ is even --> not necessarily true, consider: $$x=1$$ and $$y=4$$;

What if xy= 44 ( x=11, y= 4 or x=4 and y= 11)

Option B, C and D hold true then....i know in must be true questions picking numbers can be confusing..

EDIT- (A) Counter-example: If x = 2, then y could not be odd, because 2*(odd) will never be divisible by four. NOT ALWAYS TRUE.
(B) If x is odd, then there are no factors of 2 in x. There are two factors of 2 in x*y, since x*y is be divisible by 4. If those two factors of 2 don't come from x, they must come from y. Therefore y MUST BE divisible by four.
(C) Counter-example: x = 3 and y = 12 --- x + 12 = 15 is odd, but y/x = 12/3 = 4 is an integer. NOT ALWAYS TRUE.
(D) Counter-example: x = 12 and y = 8 ---- x + y = 20 is even, but x/y = 12/8 = 3/2 is not an integer. NOT ALWAYS TRUE.
(E) Counter-example: x = 3 and y = 4 ---- x^y = 3^4 = 81 is not even. NOT ALWAYS TRUE.
_________________

Kudos [?]: 247 [0], given: 57

Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 4554

Kudos [?]: 8959 [1], given: 111

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

25 Jun 2013, 09:20
1
KUDOS
Expert's post
prateekbhatt wrote:
What if xy= 44 ( x=11, y= 4 or x=4 and y= 11)

Option B, C and D hold true then....i know in must be true questions picking numbers can be confusing.

Dear prateekbhatt
Yes. Remember that in any "must be true" math question, one answer has to be true, and most of the time, the other four will be usually be true --- some will be things that seem like they should be true. The GMAT simply is not going to give you one 100% true choice and four 100% false choices ---- it's going to be something more like one 100% true choice and four 90-95% true choices.

This means that, for any pair of numbers you pick, unless you are exceedingly lucky (or exceedingly skillful at picking numbers), more than one answer choice will still be true. The value of any single pair of numbers lies in [b]the choices you can eliminate[/b] ---- you pick one pair, eliminate some choices, then pick another pair, eliminate more choices, and whittle down the options. You are in the wrong frame of mind if you are trying to find the idea choice so that you can eliminate the four incorrect options all at once. If that happens by chance, great, but don't spend any time striving for that. Instead, think simplicity and speed. If I were plugging in numbers for this, my first choices would be the simplest kindergarten possibilities --- x = 1 & y = 4, then x = 4 & y = 1, then x = 2 & y = 2 --- in the first 10-20 seconds, you should be able to eliminate at least three answer choices with very easy selections for the variable values. Focus speed & efficiency, not on making ideal choices. Once you are down to two answers, you may need a more sophisticated choice --- for example, none of those choices would eliminate (D), so you would have to choose something like x = 2 & y = 8. That eliminates everything except the OA, (B), and I didn't use anything other than single-digit choices. Often you can remain with single digit math and get everything you need.

Does all this make sense?
Mike
_________________

Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)

Kudos [?]: 8959 [1], given: 111

Director
Joined: 27 Jun 2011
Posts: 879

Kudos [?]: 247 [0], given: 57

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

25 Jun 2013, 21:27
mikemcgarry wrote:
prateekbhatt wrote:
What if xy= 44 ( x=11, y= 4 or x=4 and y= 11)

Option B, C and D hold true then....i know in must be true questions picking numbers can be confusing.

Dear prateekbhatt
Yes. Remember that in any "must be true" math question, one answer has to be true, and most of the time, the other four will be usually be true --- some will be things that seem like they should be true. The GMAT simply is not going to give you one 100% true choice and four 100% false choices ---- it's going to be something more like one 100% true choice and four 90-95% true choices.

This means that, for any pair of numbers you pick, unless you are exceedingly lucky (or exceedingly skillful at picking numbers), more than one answer choice will still be true. The value of any single pair of numbers lies in [b]the choices you can eliminate[/b] ---- you pick one pair, eliminate some choices, then pick another pair, eliminate more choices, and whittle down the options. You are in the wrong frame of mind if you are trying to find the idea choice so that you can eliminate the four incorrect options all at once. If that happens by chance, great, but don't spend any time striving for that. Instead, think simplicity and speed. If I were plugging in numbers for this, my first choices would be the simplest kindergarten possibilities --- x = 1 & y = 4, then x = 4 & y = 1, then x = 2 & y = 2 --- in the first 10-20 seconds, you should be able to eliminate at least three answer choices with very easy selections for the variable values. Focus speed & efficiency, not on making ideal choices. Once you are down to two answers, you may need a more sophisticated choice --- for example, none of those choices would eliminate (D), so you would have to choose something like x = 2 & y = 8. That eliminates everything except the OA, (B), and I didn't use anything other than single-digit choices. Often you can remain with single digit math and get everything you need.

Does all this make sense?
Mike

Absolutely it does!

I am gonna follow this- To start with the easy numbers and the strategy would be to eliminate 4 options rather that just looking for the right answer.
_________________

Kudos [?]: 247 [0], given: 57

Magoosh GMAT Instructor
Joined: 28 Dec 2011
Posts: 4554

Kudos [?]: 8959 [0], given: 111

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

01 Jul 2013, 13:45
For more on "must be true" math problems, see this blog article:
http://magoosh.com/gmat/2013/gmat-quant ... -problems/

Mike
_________________

Mike McGarry
Magoosh Test Prep

Education is not the filling of a pail, but the lighting of a fire. — William Butler Yeats (1865 – 1939)

Kudos [?]: 8959 [0], given: 111

Intern
Joined: 04 Dec 2012
Posts: 8

Kudos [?]: [0], given: 7

Schools: ISB '15
Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

02 Jul 2013, 07:23
Since x * y is divisible by 4 Only the following two options are possible.

1. Both x and y should have a factor of 2 in them, which means both must be even numbers.
2. In case the above scenario is false, then one of them must have 4 as a factor.

Note : The reason for the above is that 4 can be written as 4 * 1 or 2 * 2. Here, x and y should contribute to this split of 4 in some way. If we observe the two conditions i have mentioned above, it is clear that the conditions follow from the above split of 4.

Kudos [?]: [0], given: 7

Non-Human User
Joined: 09 Sep 2013
Posts: 14854

Kudos [?]: 287 [0], given: 0

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

05 Sep 2014, 11:35
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).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

Kudos [?]: 287 [0], given: 0

Non-Human User
Joined: 09 Sep 2013
Posts: 14854

Kudos [?]: 287 [0], given: 0

Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

20 Apr 2016, 03:56
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).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________

Kudos [?]: 287 [0], given: 0

Board of Directors
Status: QA & VA Forum Moderator
Joined: 11 Jun 2011
Posts: 3111

Kudos [?]: 1146 [0], given: 327

Location: India
GPA: 3.5
Re: If x and y are positive integer and xy is divisible by 4 [#permalink]

### Show Tags

20 Apr 2016, 07:55
gmatchase wrote:
If $$x$$ and $$y$$ are positive integer and $$xy$$ is divisible by 4, which of the following must be true?

A) If $$x$$ is even then $$y$$ is odd.
B) If $$x$$ is odd then $$y$$ is a multiple of 4.
C) If $$x+y$$ is odd then $$y/x$$ is not an integer.
D) If $$x+y$$ is even then $$x/y$$ is an integer.
E) $$x^y$$ is even.

One stop solution (B) just in under 10 secs !

Since $$xy$$ is divisible by 4,

1. Either x is a multiple of 4

or

2. Y is a multiple of 4
_________________

Thanks and Regards

Abhishek....

PLEASE FOLLOW THE RULES FOR POSTING IN QA AND VA FORUM AND USE SEARCH FUNCTION BEFORE POSTING NEW QUESTIONS

How to use Search Function in GMAT Club | Rules for Posting in QA forum | Writing Mathematical Formulas |Rules for Posting in VA forum | Request Expert's Reply ( VA Forum Only )

Kudos [?]: 1146 [0], given: 327

Re: If x and y are positive integer and xy is divisible by 4   [#permalink] 20 Apr 2016, 07:55
Display posts from previous: Sort by

# If x and y are positive integer and xy is divisible by 4

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.