DIGITS AND DECIMALS- Chapter 1 Number 11- Basic DS Question

[shawndx3]

Data Sufficiency: The number A is a two-digit positive integer, the number B is the two digit positive integer formed by reversing the digits of A. If Q=10B-A what is the value of Q?

1) The tens digit of A is 7
2) The tens digit of B is 6

I did it algebraically and got the right answer that Q=99y so Q is only dependent upon y... (6) So I understand why it is B

However, I was thinking, what if I had a different thought process...
What is XY?
I know X is 7 and I know Y is 6 and I know A is 10x+Y=76 and I know B is 10y +X which is 67 so could I not just simply find Q by 10(67)-76= Q

I would have then assumed that Both answers are needed to yield Q... I could have easily gone this route and was lucky I solved it algerabrically to see that X Was not needed...

Is there a way to not know how to recognize that I should have not gone the second way? This worries me as I could have easily made this mistake and not even realized it...

[george.kourdin]

i went that route..facepalm

i think the thing look out for in general and this problem is a good example of that, is that instances where answer choice C is just too easy and makes perfect sense, are likely to be questions where C is wrong. in this case, the answer is literary served to us on a platter if we go with C. must be nice right? sadly, gmat is made by people that enjoy suffering and tears. picking C is too easy and it does not cause neither suffering nor tears, therefore, its likely wrong. not always...just most of the time the obv answer is likely to be wrong

[jnelson0612]

George, that is a great point! We call that the "C trap"--both statements together give you EXACTLY what you need and you can get the answer without doing any work. Be very wary of these situations!