* Slight Data Sufficiency Confusion

[mobenny]

Given
If :xy>0

Question
Does (x-1)(y-1)=1

Statements:
1)x+y=xy
2)x=y

I got confused on this problem as to how I am allowed to use the information the first statement. I was not sure if I could substitute x+y for xy in the question. When I looked back in the answer I say that this was actually the correct method to solve this problem.

I just wanted to make sure because this seems like a crutical point to DS questions. So, can you use the information in the statements to test the question?

Thanks,

Moses

[africanchallenge]

Why don't you just simplify the question?
I would say: is xy-x-y+1=1? that is is xy=x+y?

1) statment 1 says it, so Sufficient.

2) Statment 2 says x=y, not suufficient ( if x=y=2, then xy=x+y, and if x not =2, xy not =x+y)

Anwser is A then....

[esledge]

Hi Moses, can you please cite the author of the question? For copyright reasons, we must have an author/source cited or we will have to delete the thread.

To answer your more general question: Can you plug from the statement (fact) into the question to test? Sure, but do so cautiously. For algebra questions, I mean be cautious about keeping a big question mark with the question in every step of your plugging and checking!

Fact (1): x+y = xy

Does (x-1)(y-1) = 1?
Does xy-x-y+1 = 1?
Does (x+y)-x-y+1 = 1? (here's where we substituted)
Does 1 = 1?
Yes, 1 = 1. Sufficient.

That said, I agree with africanchallenge: better to rephrase the question completely before evaluating the statements. You'll usually save yourself some work by doing so.

[mobenny]

Thank you for your response. The author is Manhattan Gmat for the question. I think I am still coming across some difficulty with Data Sufficiency questions. I don't clearly understand how to answer questions that involve lots of variables. For most DS questions I rely heavily on intuition and logic. I would like to develop some skills for answering this quesion type. For example, I understand that I should always look to rephrase first. And, if my rephrased question matches the statment than the statement is suffiecient. Rephrasing is not always possible and I would like to know how to handle these questions as well. Regarding plugging in the statement into the question. If I plug the statement into the question and the question is True, then the statement is sufficient, correct? Or, do you recommed an alternative method for solving DS questions where rephrasing is not possible or not apparent?

Can you please forward me to some literature online regaring DS quesions on the GMAT.

Thank you,

Moses

[RonPurewal]

Thank you for your response. The author is Manhattan Gmat for the question. I think I am still coming across some difficulty with Data Sufficiency questions. I don't clearly understand how to answer questions that involve lots of variables. For most DS questions I rely heavily on intuition and logic. I would like to develop some skills for answering this quesion type. For example, I understand that I should always look to rephrase first. And, if my rephrased question matches the statment than the statement is suffiecient. Rephrasing is not always possible and I would like to know how to handle these questions as well. Regarding plugging in the statement into the question. If I plug the statement into the question and the question is True, then the statement is sufficient, correct? Or, do you recommed an alternative method for solving DS questions where rephrasing is not possible or not apparent?

Can you please forward me to some literature online regaring DS quesions on the GMAT.

Thank you,

Moses

well, you're not going to find a single, one-stop-shopping method for solving DS questions IN GENERAL.
since DS questions span all math subject areas, as well as all difficulty levels, it shouldn't surprise you to find out that there won't be one single, solitary method for solving them.

--

nevertheless, you may find that the following HIERARCHY OF SOLUTION METHODS helps you out:

1. can i find a "textbook method" to solve the problem?
- i.e., can i find an 'opener' that might be in an answer key, etc.?

IF NOT,
2. can i "plug in numbers"?
- on data sufficiency, you have to plug in MULTIPLE numbers or sets of numbers, with the goal being to TRY FOR 'INSUFFICIENT'.
in other words, you should try multiple values, with your goal being to get both a 'yes' and a 'no' (if it's a yes/no question) or two different values (if it's a value question).
if it's a number properties problem, you should make sure that you figure out the TYPES of numbers being tested, and plug in accordingly. for instance, if the problem involves absolute values, then it's likely that you'll have to plug in both positive and negative numbers.

IF NOT,
3. can i use the "easy statement", or perform PROCESS OF ELIMINATION in any other way?
- always use the "easy statement" first, if there is one, and choose the grid (ad/bce or bd/ace) accordingly.
ironically, the hardest problems commonly have one very "easy" statement out of the two (such as a statement that doesn't contain one of the relevant variables).

IF NOT,
4. guess
- if you get to this point, DO NOT DELIBERATE - just guess. remember that any random guess is as good as any other random guess. if you're an extreme "deliberator", then, as weird as it may sound, you may want to have an actual method for random guessing. this could be as simple as "pick the first one of the remaining choices", but anything you can do to prevent staring at the problem is good.

again, when you move through the above hierarchy, you need to progress EFFICIENTLY through the different steps. in other words, if one of the steps isn't working for you, you should more or less IMMEDIATELY move on to the next step. i like to follow a "20 second rule": if you stare at a problem for 20 seconds without doing anything, then it's time to move on to the next step of the hierarchy.
this may be difficult to implement at first, especially if you're a "deliberator" by nature, but it's ABSOLUTELY ESSENTIAL for your success on this exam. you will not be able to finish the quant section on time if you aren't adaptable and don't move quickly from one method of problem solving to another.

[RonPurewal]

and as for "literature" ... again, data sufficiency is much too broad of a topic, unless you're looking for the VERY BASICS of the genre (which you probably understand already, if you've solved more than a few of them).

nevertheless, one good place to start is these forums!
go to the GMATPREP MATH section in particular, and go through the reams and reams and reams of available DS questions.