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.
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