If x, y, and z are positive integers, x is a factor of 2y

We are not given that z=3y, we are given that z=3y*integer (3y is a factor of z). Now, if we substitute z in \(\frac{zx}{3y}\), we'l get: \(\frac{zx}{3y}=\frac{(3y*integer)*x}{3y}=integer*x=integer\).

Hope it helps.

if we take x=4, y=2 and z=12...
x(=2) is factor of 2y(=4)
and 3y(=6) is a factor of z(=12)......
D it is.......

I have a doubt here. Option E is also possible.
The answer option is
zy/3x
z is divisible by 3y. that means z/3 will always yield an integer.
y/x will also give an integer.
When we multiply these 2 the result should be an integer as all x,y,z are integers. Please clarify if I am missing something.

You are not reading the question carefully.

The question asks: which of the following MUST also be an integer? Not COULD also be an integer?

Ok.. I get your point. Can you give me an example where E would not be an integer?

Yes.

\(x\) is a factor of \(2y\), means that \(\frac{2y}{x}=integer\). Say y = 2 and x = 1.

\(3y\) is a factor of \(z\), means that \(\frac{z}{3y}=integer\). Say z = 3 and y = 1.

E) \(\frac{zy}{3x}=\frac{3*1}{3*2}=\frac{1}{2} \neq integer\).

Hope it helps.

Hi All,

I'm a big fan of TESTing VALUES (which is showcased in several of the explanations) - it's perfect for these types of questions. In addition, the answer choices are written in such a way that you can answer this question using Number Property rules....

In the prompt, we're told that "3Y is a factor of Z." This is just a wordy way of saying Z/3Y is an INTEGER. We're also told that X, Y and Z are positive integers.

Looking at the answers, what do you notice about answer D?

ZX/3Y....

It's interesting that the letters in in the numerator are NOT in alphabetical order. I wonder why THAT is? Is it to hide the fact that this answer has Z/3Y in it? We already know that Z/3Y is an integer and we know that X is an integer too.

Answer D literally translates into (integer)(integer)....which is ALWAYS an integer.

Final Answer:

GMAT assassins aren't born, they're made,
Rich

Solution

• Given: x is factor of 2y.


o Thus, we can write \(2y = k * x\), where k is an integer

o \(\frac{x}{y} = \frac{2}{k}\)

• Given: 3y is a factor of z.


o Thus, we can write \(z = q * 3y\), where q is an integer

o \(\frac{y}{z} = \frac{1}{3q}\)

• Thus \(x : y : z = 2: k: 3qk\)

Now let us consider the options one by one and check which of the following MUST be an integer

1. \(\frac{y}{x} = \frac{k}{2}\)

a. We cannot be sure, since we do not know the value of k

2. \(\frac{2y}{6} = \frac{k}{3}\)

a. Again, we cannot be sure, since we do not know the value of k

3. \(\frac{xy}{3z} = \frac{2k}{9qk} = \frac{2}{9q}\)

a. Again, we cannot be sure, since we do not know the value of q

4. \(\frac{zx}{3y} = \frac{3qk*2}{3k} = 2q\)

a. This has to be an integer, since 2q is an integer

At this point, we can stop and mark the answer as D, but just to be sure, let’s check the last option.

5. \(\frac{zy}{3x} = \frac{3qk*k}{3*2}= \frac{qk^2}{2}\)

a. We cannot be sure this is an integer, since we do not know the value of k or q.

Correct Answer: Option D

Regards,
Saquib
Quant Expert
e-GMAT

Why have you multiplied both sides by x integer?

Thank you

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