That is what I call a "gotcha" question. It is a simple math question, except they catch people by a syntax. I wonder what percentage of people got it right the first time.
Yes, good description- it's a very easy trap to fall into. And if they'd used the notation f(x), it's likely no one would fall into the trap- instead they wrote a box around each number to express the function.
There's another question in GMATPrep which does have the wrong answer, which I'll dig out of my notes at some point- don't have the details handy.
GMAT Tutor in Toronto
If you are looking for online GMAT math tutoring, or if you are interested in buying my advanced Quant books and problem sets, please contact me at ianstewartgmat at gmail.com