Moderately clever. Pretty transparent once you know what to look for, of course.
Yeah, it irritated me for about five minutes (and 10 correct answers in a row) before I figured it out.
Not freaky really. Using this kind of equation there are only very few symbols you can end up with. For example 10 = (10-1)=9, just like 19=(19-10)=9. The app seems to generate symbols in such a way that the only possible solutions are the symbol that is (shockingly) the answer. The app shuffles the symbols every time you restart to make you think that you are picking something different every time.
Yeah…this is pretty cute. These kinds of math based tricks are pretty boring once you get the basic drift though.