What is the value of l + 2t ?

Dear vikasp99,

I'm happy to respond.

This is a question written by someone who knows a lot about math but not a lot about question design. Thus, it is not a good GMAT question, despite the fact that it is mathematically well designed.

You see, first of all, each statement alone is insufficient. In general, if we want the value of one linear combination of two variables, the value of any other linear combination that's not a multiple will not be enough.

The person who created the question was mathematically astute enough to design it so that:
(l + t) + (l + 3t) = 2*(l + 2t)
That's very clever.

Nevertheless, almost every student beyond the beginning stages know that one equation is not enough to solve for two variables, but two equations usually are enough to solve for one variable. In other words, folks will realize that (C) is the answer just applying one of the most basic algebra rules, a rule that isn't even 100% correct. In other words, a student can be entirely unaware of the algebraic elegance above and still solve the problem.

This is problematic as a question. It would work as an easy question (very few students would get it wrong). It makes the kind of design mistakes that simply don't appear in official GMAT questions. In an official GMAT question, if there's a elegant algebraic move, then seeing that move is either necessary to solve the problem or the only way to avoid prohibitively long calculation. In other words, for solving the question correctly in a limited amount of time, seeing the elegant algebraic move would be absolutely essential. Here, seeing it is absolutely irrelevant.

Once again, the question author apparently knew a good deal of math, but did not know much about question design. BTW, I have no idea why they chose such inconvenient letters.

Does all this make sense?
Mike