DS Problem

[beatthegmat]

Posting on behalf of galit...

hi, my name is galit, I am new here, wanted to ask a question which is taken from the princeton review book:

if x and y are positive integers, is x a factor of 12?

1.the product xy is a factor 12

2.y=3

the answer is a... I don't understand why... is there any other way for solving it instead of plugging in values? ... also, if anyone can explain me all this stuff about factors and this stuff...I would reeeeally appreciate this... desperate ...well not that so...galit...

[Stuart@KaplanGMAT]

Posting on behalf of galit...

hi, my name is galit, I am new here, wanted to ask a question which is taken from the princeton review book:

if x and y are positive integers, is x a factor of 12?

1.the product xy is a factor 12

2.y=3

When solving DS number property questions, we generally have two options: use our knowledge of number property rules or plug in numbers.

If you read a question and/or statement and the rules jump out at you, then using them will almost certainly be the most efficient approach; if they don't jump out at you, then you can't afford to waste time staring at the screen hoping for inspiration to strike from above - you have to either get your hands dirty and plug in numbers or guess and move on.

This question is a perfect example.

We know that x and y are positive integers - we don't know anything else.

Q: is x a factor of 12? In other words, is 12 a multiple of x? Or, another way we could phrase it, is 12/x an integer? It's the 3rd way of phrasing it that will help us quickly answer this question.

(1) xy is a factor of 12.

So, 12/xy = integer. Multiplying both sides by y, we get:

12/x = y*integer. Since y is an integer, y*int is also an integer.

In other words, 12/x is an integer - the definition of a factor. Therefore, x is definitely a factor of 12: sufficient.

(2) y=3. No information about x: insufficient.

(1) is sufficient, (2) isn't: choose (a).

[[email protected]]

I tried doing the problem by subsituition,

Case 1

x=4 and y=3

In this case xy =12 and is a factor of 12.

But still x=4 is not a factor of 12.

Case 2

x= 12 y =2

xy =24

x =12 is a factor of 12

Therfore A is insufficient

Please explain

[Stuart@KaplanGMAT]

I tried doing the problem by subsituition,

But still x=4 is not a factor of 12.


Please explain

Explanation:

umm.. 4 IS a factor of 12!