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...

yes, just be prepared for the GMAT to try to trick you with this type of thing, and always pay very careful attention to what is actually being asked. remember that if you are asked for some combination of two variables you very often don't actually need both of them to answer the question. i hope this helps; let me know if i need to go into greater detail..

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Tim Sanders
Manhattan GMAT Instructor