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...

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

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!

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Jamie Nelson
ManhattanGMAT Instructor