kevind147 wrote:If X/Y = 2/3, what is the value of (X-Y)/X?
A. -1/2
B. -1/3
C. 1/3
D. 1/2
E. 5/2
The answer in the book gives a long drawn-out explanation, but can you not just assume that X = 2 and Y = 3 from the equation provided? So (2-3)/2 = -1/2. Is this assumption correct?
It's useful to notice here that
every answer choice is an exact number. That
must mean that the answer is the same for any x and y you choose, as long as x/y = 2/3. Otherwise the question would have more than one right answer, and of course that can't happen on the GMAT. It is indeed perfectly fine to plug in x = 2 and y = 3 here; whatever answer you get must be the right answer.
You could
not, however, reliably use this strategy if the question looked like this:
If X/Y = 2/3, what is the value of (X-Y)/X?
A. -1/2
B. -1/3
C. 1/3
D. 1/2
E. It cannot be determined from the information given.
Because we have answer choice E here, we can't be sure that we will always get the same answer for any x and y we choose. By plugging in x=2 and y=3, you can be sure that the answer is
either A or E, but you'd need to do more work after that to choose between them.
That's a long way of saying: whether picking numbers is a reliable strategy depends on the answer choices. As a caution, take a look at question 181 in the OG (11th ed. pg. 176-177) -- here, answer choice E says 'It cannot be determined...", and you won't get the right answer if you just plug in one set of numbers. Plugging in numbers can be a reliable strategy if you know when you can use it, and you know when you can't.
