i get 820 as well...is this incorrect?

I do not know exactly if GMATprep throws some "experimental" questions at you - that is, in this case, questions that do not count, since there's no way to collect the data from the experiment. That could very well be true, since such an easy question would not pop up unless you're doing horribly.

What I will say is this: do not worry about the difficulty level of questions on the big day. I remember I started off with some decent to easy questions, then around 10 or 15 it got hard, but I occasionally got the extremely easy question, something with powers that was pretty basic. I did not fret and the result was a positive one: got 50 in quant.

Thank DanaJ; I am not worrying about difficulty level anymore, at least not with Q. This was just a thought during the test & since now that I know how GMAT could throw some basic questions even when I am doing good my confidence during the test will slightly be more.

And money9111: Yes, it is correct answer but, my point was just to show the type of question under the circumstance, not to find out answer.